Get rid of some no-longer-necessary casts.

g_ascii_isXXX() can be handed a char value safely; they won't try to use
it as an index into a table before checking it.

Change-Id: Icf80163fb5f9dccd0b97917e318b1db0bf3e0cf8
Reviewed-on: https://code.wireshark.org/review/4801
Reviewed-by: Guy Harris <guy@alum.mit.edu>