Merge bitcoin/bitcoin#23954: doc: remove CC_FOR_BUILD from OpenBSD build doc

62c173a doc: remove CC_FOR_BUILD from OpenBSD build doc (fanquake)

Pull request description:

  This is no-longer necessary after upstream libsecp256k1 changes that have been pulled into our repo.

ACKs for top commit:
  theStack:
    ACK 62c173a

Tree-SHA512: 7c328185afec366ff49096512b2fcc3c2f2a2381875f65f5444ffacf118f9a51e4e2237daaae6b32f742a7c4c54c1fbba7cee4e1807d7c4816123f6b0e7c33c8

Author: MarcoFalke

doc/build-openbsd.md

@@ -25,8 +25,8 @@ See [dependencies.md](dependencies.md) for a complete overview.
 **Important**: From OpenBSD 6.2 onwards a C++11-supporting clang compiler is
 part of the base image, and while building it is necessary to make sure that
 this compiler is used and not ancient g++ 4.2.1. This is done by appending
-`CC=cc CC_FOR_BUILD=cc CXX=c++` to configuration commands. Mixing different
-compilers within the same executable will result in errors.
+`CC=cc CXX=c++` to configuration commands. Mixing different compilers within
+the same executable will result in errors.
 
 ### Building BerkeleyDB
 
@@ -84,7 +84,7 @@ To configure with wallet:
 
 To configure without wallet:
 ```bash
-./configure --disable-wallet --with-gui=no --disable-external-signer CC=cc CC_FOR_BUILD=cc CXX=c++ MAKE=gmake
+./configure --disable-wallet --with-gui=no --disable-external-signer CC=cc CXX=c++ MAKE=gmake
 ```
 
 To configure with GUI: