aarch64: Handle fewer relocations for RTLD_BOOTSTRAP

The RTLD_BOOTSTRAP branch is used to relocate ld.so itself.  It only
needs to handle RELATIVE, GLOB_DAT, and JUMP_SLOT.
TLSDESC/TLS_DTPMOD/TLS_DTPREL handling can be removed.  Remove
`case AARCH64_R(RELATIVE)` as well as elf_machine_rela has checked it.

Tested on aarch64-linux-gnu.