tests/subsys/llext: Remove llext.simple.readonly_fs_loader

The fs_loader test is always executed in the main test source when
CONFIG_FILE_SYSTEM is set, and this is in turn enabled by per-board
config overlays. So there is no point in having a separate test case:
it is simply a duplicate of the "readonly" test.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>