redshift: Don't remove icon-theme.cache

This is done already in the setup hook of hicolor-icon-theme itself, so
no need to do it again.

Since the removal in the redshift derivation is in postFixup (as opposed
to the removal done by the setup hook in preFixup) and it's also not
using the -f flag of rm, the build fails.

Signed-off-by: aszlig <aszlig@nix.build>