regex.3: Remove unnecessary include

POSIX requires that the <regex.h> header shall define
the structures and symbolic constants used by the
regcomp(),  regexec(), regerror(), and regfree() functions.

Therefore, there should be no need to include <sys/types.h>
at all.

The POSIX docs don't use that include:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/regcomp.html

Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/regex.3

@@ -32,7 +32,6 @@
 regcomp, regexec, regerror, regfree \- POSIX regex functions
 .SH SYNOPSIS
 .nf
-.B #include <sys/types.h>
 .B #include <regex.h>
 .PP
 .BI "int regcomp(regex_t *" preg ", const char *" regex ", int " cflags );