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

include/curl/curl.h

@@ -3184,7 +3184,8 @@ typedef struct curl_version_info_data curl_version_info_data;
                                              supported */
 #define CURL_VERSION_SSPI         (1<<11) /* Built against Windows SSPI */
 #define CURL_VERSION_CONV         (1<<12) /* Character conversions supported */
-#define CURL_VERSION_CURLDEBUG    (1<<13) /* Debug memory tracking supported */
+#define CURL_VERSION_CURLDEBUG    (1<<13) /* Debug memory tracking supported
+                                             (deprecated) */
 #define CURL_VERSION_TLSAUTH_SRP  (1<<14) /* TLS-SRP auth is supported */
 #define CURL_VERSION_NTLM_WB      (1<<15) /* NTLM delegation to winbind helper
                                              is supported */