llext: remove LOADER_BUILD_ONLY code

The latest commits allow llexts to be built on any architecture, even on
those where llext loading is still unsupported, which is very useful for
testing purposes. This means that the LOADER_BUILD_ONLY logic that was
added for this purpose in 1408d1e5 is no longer needed.

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