tests/lib/fdtable/prj.conf: remove hardcoded CONFIG_COVERAGE=y

There is absolutely no other test in the entire codebase that hardcodes
this setting. I found no comment or any other explanation why this test
should be unique. So it really looks like just a glitch introduced when
this test was added by PR #17618 / commit f1afb4c2.

This was discovered in three different ways:

- COVERAGE=y adds the absolute and non-deterministic source path in
  .rodata sections
- it adds .gnco files in the build directory
- it makes (some) tests run 10 times slower:

qemu_x86_64   lib/fdtable/libraries.os.fdtable PASSED (qemu 2.086s)
qemu_x86_long lib/fdtable/libraries.os.fdtable PASSED (qemu 2.316s)
qemu_xtensa   lib/fdtable/libraries.os.fdtable PASSED (qemu 2.033s)
mps2_an385    lib/fdtable/libraries.os.fdtable PASSED (qemu *31.286s*)
qemu_x86      lib/fdtable/libraries.os.fdtable PASSED (qemu *31.862s*)

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