lavu/riscv: remove bogus B extension

The B Bit manipulation extension was not defined to this day, and
probably never will. Instead it was broken down into Zba, Zbb, Zbc and
Zbs with no particular blessed set to make up B.

This removes the bogus field test. Linux never set this bit, nor
(AFAICT) did FreeBSD or any other OS. We can always add it back in the
unlikely event that it gets taken into use.

libavutil/riscv/cpu.c

@@ -91,8 +91,6 @@ int ff_get_cpu_flags_riscv(void)
             ret |= AV_CPU_FLAG_RVF;
         if (hwcap & HWCAP_RV('D'))
             ret |= AV_CPU_FLAG_RVD;
-        if (hwcap & HWCAP_RV('B'))
-            ret |= AV_CPU_FLAG_RVB_ADDR | AV_CPU_FLAG_RVB_BASIC;
 
         /* The V extension implies all Zve* functional subsets */
         if (hwcap & HWCAP_RV('V'))