cmd/go: mod edit: add -droptoolchain flag

[Narita-1095305]

Add support for the -droptoolchain flag to go mod edit, which removes
the toolchain directive from go.mod. This complements the existing
-toolchain flag and allows users and tools to remove toolchain
constraints via CLI.

The -toolchain and -droptoolchain flags are mutually exclusive.

Fixes #73744

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Patch Set 1:

(1 comment)

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Patch Set 1:

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