regcomp.c: Rmv remaining uses of PL_Posix_ptrs

regcomp.c: Rmv remaining uses of PL_Posix_ptrs

Previous commits have removed all but a few uses of PL_Posix_ptrs.  This
removes the rest.  ASCII is the same whether over all code points, or
just the ASCII range, so we can substitute the version for all code
points.  There is an extra intersection introduced by this commit during
the construction of a synthetic start class under /a and /aa, but the
performance hit should be negligible.