llext: fix llext_find_section(), remove llext_section_by_name()

The function llext_section_by_name() is used only in one place, and it
expects the caller to have the section headers cache available. This
cache is freed after the ELF file is loaded, so the function is not
usable in the context where it is called.

Remove the function and replace the call with a direct search in the
ELF file section headers array, as was done before 08eb314c.

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