C API: Merge functionality of LTP_QUIET into LTP_REPRODUCIBLE_OUTPUT

[pevik]

LTP_REPRODUCIBLE_OUTPUT=1 environment variable was added in 5894abc5f to help valgrind to use LTP for testing. It discards the actual content of the messages printed by the test (removes everything after printing TINFO/TPASS/TFAIL/... flag).

Later LTP_QUIET=1 was added which suppresses printing TCONF, TWARN, TINFO, and TDEBUG messages.

Valgrind project uses both variables but no need to handle them separately:

https://sourceforge.net/p/valgrind/mailman/valgrind-developers/?viewmonth=202603

Sounds fine, Petr. For Valgrind testing, the functionality of
both LTP_REPRODUCIBLE_OUTPUT and LTP_QUIET is useful. But
there's no need to control the two features independently.
Merging the functionality under one controlling env var works
perfectly fine from the Valgrind testsuite perspective.

[pevik]

Part of:
https://lore.kernel.org/ltp/20260422141234.632195-1-pvorel@suse.cz/T/#t
https://patchwork.ozlabs.org/project/ltp/list/?series=501017&state=*

[pevik]

Part of:
https://lore.kernel.org/ltp/20260423084123.657690-2-pvorel@suse.cz/T/#u
https://patchwork.ozlabs.org/project/ltp/list/?series=501162&state=*