RISC-V Regression: Remove scalable compile option

RISC-V Regression: Remove scalable compile option

Since we already set scalable vectorization by default, this flag is redundant.

Also, we are start to full coverage testing with different compile option.
E.g --param=riscv-autovec-preference=fixed-vlmax.
To avoid compile option confusion. Remove it.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp: Remove scalable compile option.