Remove unused STREQ macro

Remove unused STREQ macro

This fixes the following compilation warning:

    regex.c:516:0: warning: macro "STREQ" is not used [-Wunused-macros]
     #define STREQ(s1, s2) ((strcmp (s1, s2) == 0))
     ^

* src/regex.c (STREQ): Remove unused macro.  It should have been removed
in a [4538a5e: Refactor regex character class parsing in [:name:]]
commit but was mistakenly left out.