"#if" directive around nullness removed from src/Compiler/Utilities/HashMultiMap.fs

[progressive-galib]

Description

Related: #18061 (partially addresses)

[github-actions]

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[T-Gro]

The 'not null' constraint has to be stay there (at least for net90 build), because it is mandated by the inner use of ConcurrentDictionairy for the same generic Key type.

Also, in the compiler codebase, all files come with an .fs implementation file as well as as an corresponding .fsi signature file (with the same core part of the name) => such pairs of files will have to be changed in sync.