MAINT: delete unused variables in unary logical dispatch

[charris]

Backport of #29843.

Clang complains on current main:

[2/5] Compiling C++ object numpy/_core/libloops_logical.dispatch.h_baseline.a.p/src_umath_loops_logical.dispatch.cpp.o
../numpy/_core/src/umath/loops_logical.dispatch.cpp:111:27: warning: unused variable 'is_not' [-Wunused-const-variable]
  111 |     static constexpr bool is_not    = true;
      |                           ^~~~~~
../numpy/_core/src/umath/loops_logical.dispatch.cpp:124:27: warning: unused variable 'is_not' [-Wunused-const-variable]
  124 |     static constexpr bool is_not    = false;
      |                           ^~~~~~
2 warnings generated.