Remove conditional checks for ssize_t type (#18996)

The ssize_t type is already used unconditionally in php-src code
everywhere except the main/s{n,p}printf.c files. On Windows ssize_t is
available as an alias to the SSIZE_T defined in BaseTsd.h (available in
affected files through the included windows.h in zend_config.w32.h).

This also makes the Autoconf macro PHP_CHECK_SIZEOF obsolete in favor of
the AC_CHECK_SIZEOF, which is more convenient to check for types without
the need to run the test program - omitting the cross-compilation
issues. AC_CHECK_SIZEOF once didn't provide including additional headers
(resolved in Autoconf versions after 2.13).

configure.ac

@@ -451,9 +451,6 @@ AC_CHECK_TYPES([socklen_t], [], [], [
   #endif
 ])
 
-dnl These are defined elsewhere than stdio.h.
-PHP_CHECK_SIZEOF([ssize_t], [8])
-
 dnl Check stdint types (must be after header check).
 PHP_CHECK_STDINT_TYPES