Remove unqualified ContantFunction and ZeroFunction

[masterleinad]

Deprecated in #12127.

[kronbichler]

There are some problems:

FAILED: /usr/bin/mpicxx   -DBOOST_NO_AUTO_PTR -DDEBUG -Iinclude -I/jenkins/workspace/dealii_PR-14048/include -I/jenkins/workspace/dealii_PR-14048/bundled/tbb-2018_U2/include -I/jenkins/workspace/dealii_PR-14048/bundled/boost-1.70.0/include -I/jenkins/workspace/dealii_PR-14048/bundled/umfpack/UMFPACK/Include -I/jenkins/workspace/dealii_PR-14048/bundled/umfpack/AMD/Include -I/jenkins/workspace/dealii_PR-14048/bundled/muparser_v2_3_3/include -isystem /usr/include/mpich -isystem /home/dealii/libs/parmetis-4.0.3/include -isystem /home/dealii/libs/trilinos-release-12-10-1/include -isystem /home/dealii/libs/superlu_dist_5.1.2/include -isystem /home/dealii/libs/petsc-3.7.6/include -isystem /home/dealii/libs/adolc-2.6.4-rc1/include -isystem /home/dealii/libs/assimp-4.1.0/include -isystem /home/dealii/libs/oce-OCE-0.18.2/include/oce -isystem /home/dealii/libs/p4est-2.0/FAST/include -isystem /home/dealii/libs/slepc-3.7.3/include -pedantic -fPIC -Wall -Wextra -Wmissing-braces -Woverloaded-virtual -Wpointer-arith -Wsign-compare -Wsuggest-override -Wswitch -Wsynth -Wwrite-strings  -Wno-literal-suffix -Wno-psabi -fopenmp-simd -Werror -std=c++14 -Wno-parentheses -Wno-unused-local-typedefs -O0 -ggdb -Wa,--compress-debug-sections -Og -MMD -MT examples/CMakeFiles/step-81.debug.dir/step-81/step-81.cc.o -MF examples/CMakeFiles/step-81.debug.dir/step-81/step-81.cc.o.d -o examples/CMakeFiles/step-81.debug.dir/step-81/step-81.cc.o -c /jenkins/workspace/dealii_PR-14048/examples/step-81/step-81.cc
/jenkins/workspace/dealii_PR-14048/examples/step-81/step-81.cc: In member function ‘void Step81::Maxwell<dim>::setup_system()’:
/jenkins/workspace/dealii_PR-14048/examples/step-81/step-81.cc:551:7: error: ‘ZeroFunction’ is not a member of ‘dealii’
       dealii::ZeroFunction<dim>(2 * dim),
       ^
/jenkins/workspace/dealii_PR-14048/examples/step-81/step-81.cc:551:7: note: suggested alternative:
In file included from /jenkins/workspace/dealii_PR-14048/examples/step-81/step-81.cc:28:0:
/jenkins/workspace/dealii_PR-14048/include/deal.II/base/function.h:510:9: note:   ‘dealii::Functions::ZeroFunction’
   class ZeroFunction : public ConstantFunction<dim, RangeNumberType>
         ^
/jenkins/workspace/dealii_PR-14048/examples/step-81/step-81.cc:557:7: error: ‘ZeroFunction’ is not a member of ‘dealii’
       dealii::ZeroFunction<dim>(2 * dim),
       ^
/jenkins/workspace/dealii_PR-14048/examples/step-81/step-81.cc:557:7: note: suggested alternative:
In file included from /jenkins/workspace/dealii_PR-14048/examples/step-81/step-81.cc:28:0:
/jenkins/workspace/dealii_PR-14048/include/deal.II/base/function.h:510:9: note:   ‘dealii::Functions::ZeroFunction’
   class ZeroFunction : public ConstantFunction<dim, RangeNumberType>
         ^

or, in a more compact form, for the tidy check:

/jenkins/workspace/dealii-tidy_PR-14048/examples/step-81/step-81.cc:551:7: error: no template named 'ZeroFunction' in namespace 'dealii'; did you mean 'dealii::Functions::ZeroFunction'? [clang-diagnostic-error]
/jenkins/workspace/dealii-tidy_PR-14048/examples/step-81/step-81.cc:557:7: error: no template named 'ZeroFunction' in namespace 'dealii'; did you mean 'dealii::Functions::ZeroFunction'? [clang-diagnostic-error]