INSTALL: defaults not removed when PETSC_CONFIGURE_OPTIONS sets things

[ksagiyam]

Initially reported by @colinjcotter

In the install script we set "--download-xxx" for PETSc configuration for selected packages by default, but these are not removed when PETSC_CONFIGURE_OPTIONS explicitly sets --download-xxx=/path/to/downloaded/xxx or --with-xxx-dir=/path/to/xxx. We probably need to do something like this

firedrake/scripts/firedrake-install

Line 813 in c245abc

petsc_options.discard("--download-hdf5")

for other packages.