std.Target: Remove functions that just wrap component functions.

Functions like isMinGW() and isGnuLibC() have a good reason to exist: They look
at multiple components of the target. But functions like isWasm(), isDarwin(),
isGnu(), etc only exist to save 4-8 characters. I don't think this is a good
enough reason to keep them, especially given that:

* It's not immediately obvious to a reader whether target.isDarwin() means the
  same thing as target.os.tag.isDarwin() precisely because isMinGW() and similar
  functions *do* look at multiple components.
* It's not clear where we would draw the line. The logical conclusion before
  this commit would be to also wrap Arch.isX86(), Os.Tag.isSolarish(),
  Abi.isOpenHarmony(), etc... this obviously quickly gets out of hand.
* It's nice to just have a single correct way of doing something.

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