[RISCV][NFC] Correct RISCVInstrInfoZc.td header comment - Zc* extensions no longer experimental

None of the Zc* extensions are experimental any more, so the comment was
out of date.

Author: asb

llvm/lib/Target/RISCV/RISCVInstrInfoZc.td

@@ -8,8 +8,6 @@
 ///
 /// This file describes the RISC-V instructions from the 'Zc*' compressed
 /// instruction extensions, version 1.0.3.
-/// This version is still experimental as the 'Zc*' extensions haven't been
-/// ratified yet.
 ///
 //===----------------------------------------------------------------------===//