backends/riscv: Remove unused relocations

backends/riscv: Remove unused relocations

None of these relocations were ever part of any object file.  The
GNU_VTINHERIT and GNU_VTINHERIT relocations were part of the obsolete
--gc-sections support which was never implemented for RISC-V.  The other
relocations are only used internally by libbfd during the relaxation pass
and eliminated before writing the object file.

	* riscv_reloc.def: Remove GNU_VTINHERIT, GNU_VTENTRY, RVC_LUI,
	GPREL_I, GPREL_S, TPREL_I, TPREL_S.