maint: remove gl/lib/reg*.c.diff; no longer needed

maint: remove gl/lib/reg*.c.diff; no longer needed

* gl/lib/regcomp.c.diff: Remove file, now that gnulib's
regcomp.c compiles regex.c with -Wno-unused-parameter.
* gl/lib/regex_internal.h.diff: Likewise.
* gl/lib/regex_internal.c.diff: This file induced a change to ensure
that the "Idx" type was unsigned and to remove a few "VAR < 0"
comparisons.  These days, it is probably fine to stay in sync with
gnulib/glibc's copies
of these files, so remove these patches, too.
* gl/lib/regexec.c.diff: Likewise.
Prompted by a report by Assaf Gordon and a suggestion from Paul Eggert.