[clang][deps] NFC: Remove dead code

This patch removes some dead code in the dependency scanner.

The `ModuleDeps::ImplicitModulePCMPath` member stopped being used in D131934.

The strict context hash was replaced in D129884 by hash of the canonical command line.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D142416