llext: xtensa: disable relative relocations for pre-located

When building pre-located LLEXTs of ET_DYN type (shared ELF object)
on Xtensa, all the R_XTENSA_RELATIVE relocations are already correct,
the current code actually breaks them by mobing the value from the
target address-space to a storage range address. Simply removing the
recalculation solves the issue.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>