external/firebird: Remove dead conditionals

For one, Clang appears to support __has_feature since at least 3.x times.  That
simplifies the first check, from 14955ed91b282ccbb395cb47c6d76e3b42b34748
"external/firebird: Support Clang ASan".

And for another, the second check, from 25764ffd4db0e5db6f9cc9f3da8691e607f48b83
"external/firebird: Better workaround for Clang alignment expectations", can be
simplified now that we no longer support neither Clang < 4 on Linux (since
685aca47da835e80f34b295c5d6389df03d1a8c2 "Bump (Linux) Clang baseline to 5.0.2")
nor Xcode < 9 on macOS (since b4f666f2e677b05cab8395fe7972b45b15f60c3f "Bump
Xcode baseline to 9.3").

But that means we are always setting USE_ASAN when building with Clang now (to
work around certain Clang alignment expectations, regardless of whether or not
we build with -fsanitize=address).  Nevertheless, keep the
__has_feature(address_sanitizer) check as a comment, to make this a bit clearer.

Change-Id: Idc9720bd763bc1e00585773ef096202e2a8a6a59
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/89162
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

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