x/pkgsite: remove -gorepo flag

[matloob]

There are two ways of setting the goroot to use for stdlib packages: one is the -gorepo flag, and the other is the GOROOT of the go command in PATH. Most of our tools use the GOROOT of the go command, so pkgsite should probably standardize on doing that.

[gabyhelp]

Related Issues and Documentation

• x/pkgsite: -gorepo flag does not work #68533 (closed)

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[cagedmantis]

cc @golang/pkgsite