Remove unused HAVE_PHPDBG (#14864)

This is not defined on Windows and isn't used. Also phpdbg doesn't
install any headers to the public usage.

UPGRADING.INTERNALS

@@ -141,6 +141,7 @@ PHP 8.4 INTERNALS UPGRADE NOTES
    - Symbols PHP_FPM_SYSTEMD, PHP_FPM_USER, and PHP_FPM_GROUP removed.
    - Symbol PTHREADS has been removed.
    - Symbol HAVE_STRPTIME_DECL_FAILS has been removed (use HAVE_DECL_STRPTIME).
+   - Symbol HAVE_PHPDBG has been removed.
    - Symbols PHP_HAVE_AVX512_SUPPORTS and PHP_HAVE_AVX512_VBMI_SUPPORTS are now
      either defined to 1 or undefined.
    - M4 macro PHP_DEFINE (atomic includes) removed (use AC_DEFINE and config.h).

sapi/phpdbg/config.m4

@@ -22,7 +22,6 @@ PHP_ARG_ENABLE([phpdbg-readline],
 
 if test "$PHP_PHPDBG" != "no"; then
   AC_HEADER_TIOCGWINSZ
-  AC_DEFINE(HAVE_PHPDBG, 1, [ ])
 
   PHP_PHPDBG_CFLAGS="-DZEND_ENABLE_STATIC_TSRMLS_CACHE=1"
   AS_VAR_IF([PHP_PHPDBG_DEBUG], [no],,