[docs][RISCV] Remove Zicond from release notes now it was backported

Zicond's graduation to non-experimental was backported to 18.x in #80018,
so remove the release note.

Author: asb

llvm/docs/ReleaseNotes.rst

@@ -94,7 +94,6 @@ Changes to the PowerPC Backend
 Changes to the RISC-V Backend
 -----------------------------
 
-* Support for the Zicond extension is no longer experimental.
 * Added assembler/disassembler support for the experimental Zabha (Byte and Halfword Atomic Memory Operations) extension.
 
 Changes to the WebAssembly Backend