Remove Xbitmanip from README

README.md

@@ -53,20 +53,6 @@ Spike supports the following RISC-V ISA features:
   - Zvfbfmin extension, v0.6
   - Zvfbfwma extension, v0.6
 
-As a Spike extension, the remainder of the proposed
-[Bit-Manipulation Extensions](https://github.com/riscv/riscv-bitmanip)
-is provided under the Spike-custom extension name _Xbitmanip_.
-These instructions (and, of course, the extension name) are not RISC-V
-standards.
-
-These proposed bit-manipulation extensions can be split into further
-groups: Zbp, Zbs, Zbe, Zbf, Zbc, Zbm, Zbr, Zbt. Note that Zbc is
-ratified, but the original proposal contained some extra instructions
-(64-bit carryless multiplies) which are captured here.
-
-To enable these extensions individually, use the Spike-custom
-extension names _XZbp_, _XZbs_, _XZbc_, and so on.
-
 Versioning and APIs
 -------------------