intel_adsp_ace15_mtpm.conf: replace blanket _STUBS with focused RTC MOCK

CONFIG_COMP_STUBS=y was enabled in thesofproject#8722 / commit 8e34109 ("AEC:
Enable Google AEC with Mock compliation").

CONFIG_COMP_STUBS indirectly enables
CONFIG_GOOGLE_RTC_AUDIO_PROCESSING_MOCK which was the desired
effect. However it also automatically and silently "mocks" all other 3rd
party modules which is not desirable. So, replace it with the more
focused `CONFIG_GOOGLE_RTC_AUDIO_PROCESSING_MOCK`. `src/audio/Kconfig`
says "CONFIG_STUBS: This should only be used in testing environments
like fuzzers or CI."

Official sof-bin releases include `google_rtc_audio_processing_mock.c`
because the CI that uses it can't use extra CONFIGS. That's another
topic for another day, see thesofproject#9410.

build-mtl/zephyr.strip is identical before versus after this commit.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

app/boards/intel_adsp_ace15_mtpm.conf

@@ -104,7 +104,9 @@ CONFIG_PROBE=y
 CONFIG_PROBE_DMA_MAX=2
 CONFIG_LOG_TIMESTAMP_64BIT=y
 
+# This mock is part of official sof-bin releases because the CI that
+# tests it can't use extra CONFIGs. See #9410, #8722 and #9386
 CONFIG_COMP_GOOGLE_RTC_AUDIO_PROCESSING=y
-CONFIG_COMP_STUBS=y
+CONFIG_GOOGLE_RTC_AUDIO_PROCESSING_MOCK=y
 
 CONFIG_KCPS_DYNAMIC_CLOCK_CONTROL=y