llext: avoid redundant 'ldr_parm' checks

Since commit 0aa6b1c9, the 'ldr_parm' pointer is guaranteed to be
valid inside all the functions of the llext_load() call tree.

This commit fixes the only exception of llext_copy_strings(), which was
not passed the 'ldr_parm' pointer, and remove the redundant checks.

No functional change is intended by this commit.

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