Mk/bsd.default-versions.mk: bump LLVM_DEFAULT to 15

• graphics/mesa* switched to LLVM version 15 some time ago
• desktop systems suffer from requiring many different LLVM versions
• having the default at something recent combined with the version matching of llvm.mk (prefering $LLVM_DEFAULT) this should lessen the number of LLVM installations on normal systems again alot.

• while here, remove outdated comment. x11@ switched to setting the LLVM version explicitely as the default was way out of date.

PR: 263456
Exp-run by: antoine