RISC-V: Remove testcase that cannot be compiled because VLEN limitation

Since the latter patch (https://gcc.gnu.org/pipermail/gcc-patches/2023-July/624689.html)
forbidden VLEN > 4096, the testcase attribute-20.c is no long need. This is obvious.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/attribute-20.c: Removed.