RISC-V: Remove R_RISCV_GNU_VTINHERIT/R_RISCV_GNU_VTENTRY

They were legacy relocation types copied from other ports.  The related
-fvtable-gc was removed from GCC in 2003.

The associated assembler directives (.vtable_inherit and .vtable_entry)
have never been supported by the RISC-V port.  Remove related ld code.

Link: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/323