doc: remove llvm-config search from configure documentation

doc: remove llvm-config search from configure documentation

As of 4dd29b6833, we no longer attempt to locate any other llvm-config
variant than plain llvm-config in configure-based builds; update the
documentation accordingly. (For Meson-based builds, we still use Meson's
LLVMDependencyConfigTool [0], which runs through a set of possible
suffixes [1], so no need to update the documentation there.)

[0]: https://github.com/mesonbuild/meson/blob/7d28ff29396f9d7043204de8ddc52226b9903811/mesonbuild/dependencies/dev.py#L184
[1]: https://github.com/mesonbuild/meson/blob/7d28ff29396f9d7043204de8ddc52226b9903811/mesonbuild/environment.py#L183

Author: Ole Peder Brandtzæg <olebra@samfundet.no>
Discussion: https://www.postgresql.org/message-id/20240518224601.gtisttjerylukjr5%40samfundet.no