pp.c: Use built-in case tables for ords < 256

pp.c: Use built-in case tables for ords < 256

Previously, all case changing on utf8-encoded strings used the tables on
disk, under the off-chance that there was a user-defined case change
override in effect.  Now that that feature has been removed, this can't
happen, so we can use the existing built-in tables.

This code has been present and ifdef'd out since 5.10.1.  New compiler
warnings forced a few other changes besides removing the #if statements

Running some primitive benchmarks showed that this sped up upper-casing of
utf8 strings in the latin1 range by 2 orders of magnitude.