configure: remove check for regex.h

Introduced by:
commit 542039fa
    Fully support mingw builds

Made redundant by:
commit ec8a2d03
    regex: gnulib guarantees that we have regex support
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

configure.ac

@@ -360,7 +360,6 @@ AC_CHECK_HEADERS([\
   net/ethernet.h \
   netinet/tcp.h \
   pwd.h \
-  regex.h \
   stdarg.h \
   syslog.h \
   sys/mount.h \