[Driver][Linux] Remove D.Dir+"/../lib" from default search paths for LLVM_ENABLE_RUNTIMES builds

The rule was added in 2014 to support -stdlib=libc++ and -lc++ without
specifying -L, when D.Dir is not a well-known system library directory like
/usr/lib /usr/lib64. This rule turns out to get in the way with (-m32 for
64-bit clang) or (-m64 for 32-bit clang) for Gentoo :
https://github.com/llvm/llvm-project/issues/54515

Nowadays LLVM_ENABLE_RUNTIMES is the only recommended way building libc++ and
LLVM_ENABLE_PROJECTS=libc++ is deprecated. LLVM_ENABLE_RUNTIMES builds libc++
in D.Dir+"/../lib/${triple}/". The rule is unneeded. Also reverts D108286.

Gentoo uses a modified LLVM_ENABLE_RUNTIMES that installs libc++.so in
well-known paths like /usr/lib64 and /usr/lib which are already covered by
nearby search paths.

Implication: if a downstream package needs something like -lLLVM-15git and uses
libLLVM-15git.so not in a well-known path, it needs to supply -L
D.Dir+"/../lib" explicitly (e.g. via LLVMConfig.cmake), instead of relying on
the previous default search path.

Reviewed By: mgorny

Differential Revision: https://reviews.llvm.org/D122444

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