Remove ld.config.txt from prebuilts build rule

Starting from the P VNDK snapshot, the linker config file will be
built and installed from the source tree at the time the VNDK
snapshot is installed to the system.img (of Q+), rather than
installing the prebuilt from /prebuilts/vndk/v{VER}. The installed
linker config file is used for both system and vendor processes,
thus should be built from the most up-to-date ld.config.txt
template in the source tree of the system partition.

Test: python gen_buildfiles.py 28 -vv
Test: m -j vndk_v28_arm64; check /system/etc/ld.config.28.txt
Bug: 74658756
Merged-In: I86034e6bbbab6a096814389332c26920ef7d40fc
Change-Id: I86034e6bbbab6a096814389332c26920ef7d40fc
(cherry picked from commit e020f713)