build: merge TrackMemory (CURLDEBUG) into debug-enabled option

Drop separate `TrackMemory` (aka `CURLDEBUG`) debug feature.

After recent changes (thread-safety,
193cb00ce9, and updates leading up to
it), `TrackMemory` is unlikely to cause build or runtime issues.

To simplify builds and debug options, enable `TrackMemory`
unconditionally for debug-enabled (aka `DEBUGBUILD`) builds. Before
this patch, this was already the default, with an option to disable
it, or enable it in non-debug-enabled builds.

Note, in practice these two debug options already went hand in hand. It
was not possible to toggle them separately for a long time due to bugs,
before 59dc9f7e69 (2024-05-28) fixed it.

This patch also removes/deprecates separate knobs and feature flags for
`TrackMemory`:
- autotools: `--enable-curldebug`/`--disable-curldebug`
- cmake: `-DENABLE_CURLDEBUG=ON`/`OFF`
- C macro: `CURLDEBUG`
- libcurl: `CURL_VERSION_CURLDEBUG` symbol deprecated in favor
  of `CURL_VERSION_DEBUG`. They always return the same value after this
  patch.

Also:
- drop `TrackMemory` from `curl -V` output.
- rename internal `CURLDEBUG` macro to `CURL_MEMDEBUG` internally.
  To avoid confusion with `DEBUGBUILD`, but to keep guarding
  `TrackMemory`-related internals for readability.
- runtests: bind `TrackMemory` to debug feature. Keep it a separate
  test feature requirement, for clarity.
- CI: drop test builds for combinations of the two options.
- GHA/linux: no longer disable TrackMemory in the TSAN job.

Ref: https://github.com/curl/curl/pull/20328#issuecomment-3754528407

Closes #20331

configure.ac

@@ -48,8 +48,6 @@ AM_CONDITIONAL(DEBUGBUILD, test "$want_debug" = "yes")
 CURL_CHECK_OPTION_OPTIMIZE
 CURL_CHECK_OPTION_WARNINGS
 CURL_CHECK_OPTION_WERROR
-CURL_CHECK_OPTION_CURLDEBUG
-AM_CONDITIONAL(CURLDEBUG, test "$want_curldebug" = "yes")
 CURL_CHECK_OPTION_SYMBOL_HIDING
 CURL_CHECK_OPTION_ARES
 CURL_CHECK_OPTION_RT
@@ -5312,9 +5310,6 @@ fi
 if test "$want_debug" = "yes"; then
   SUPPORT_FEATURES="$SUPPORT_FEATURES Debug"
 fi
-if test "$want_curldebug" = "yes"; then
-  SUPPORT_FEATURES="$SUPPORT_FEATURES TrackMemory"
-fi
 if test -n "$CURL_CA_EMBED"; then
   SUPPORT_FEATURES="$SUPPORT_FEATURES CAcert"
   CURL_CA_EMBED_msg="$CURL_CA_EMBED"
@@ -5453,11 +5448,6 @@ XC_CHECK_BUILD_FLAGS
 SSL_BACKENDS=${ssl_backends}
 AC_SUBST(SSL_BACKENDS)
 
-if test "$want_curldebug_assumed" = "yes" &&
-   test "$want_curldebug" = "yes" && test "$USE_ARES" = "1"; then
-  ac_configure_args="$ac_configure_args --enable-curldebug"
-fi
-
 CURL_PREPARE_CONFIGUREHELP_PM
 
 AC_CONFIG_FILES([\