cmd/go/internal/modload: remove outdated comments and redundant tests

The outdated comment in modfile.go was missed in CL 315409.

Upon a closer look at the test case in mod_go_version_vendor.txt, it
is almost completely redundant with the new test in
mod_vendor_goversion.txt. Make it completely redundant and remove it.

Updates #36876

Change-Id: Ibcd1f6b426460aaafbd6dc0be93078547904572b
Reviewed-on: https://go-review.googlesource.com/c/go/+/316209
Trust: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>

src/cmd/go/internal/modload/modfile.go

@@ -501,9 +501,6 @@ func goModSummary(m module.Version) (*modFileSummary, error) {
 		// return the full list of modules from modules.txt.
 		readVendorList()
 
-		// TODO(#36876): Load the "go" version from vendor/modules.txt and store it
-		// in rawGoVersion with the appropriate key.
-
 		// We don't know what versions the vendored module actually relies on,
 		// so assume that it requires everything.
 		summary.require = vendorList

src/cmd/go/testdata/script/mod_vendor_goversion.txt

@@ -11,25 +11,40 @@ go build example.net/need117
 stderr '^bad114[/\\]bad114.go:15:2: duplicate method Y$'
 
 
-# With a vendor/modules.txt lacking language versions, the world is topsy-turvy.
-# Things that ought to build shouldn't, and things that shouldn't build do.
+# With a vendor/modules.txt lacking language versions, the world is topsy-turvy,
+# because we have to guess a uniform version for everything.
+#
+# We always guess Go 1.16, because that was the last version for which
+# 'go mod vendor' failed to record dependency versions, and it has most of
+# the language features added since modules were introduced in Go 1.11.
+#
+# Even so, modules that declare 'go 1.17' and use 1.17 features spuriously fail
+# to build, and modules that declare an older version and use features from a
+# newer one spuriously build (instead of failing as they ought to).
 
 go mod vendor
-go build example.net/bad114
+
+! grep 1.17 vendor/modules.txt
 ! go build example.net/need117
 stderr '^vendor[/\\]example\.net[/\\]need117[/\\]need117.go:5:18: .*\n\tconversion of slices to array pointers only supported as of -lang=go1\.17'
 
+! grep 1.13 vendor/modules.txt
+go build example.net/bad114
+
 
 # Upgrading the main module to 1.17 adds version annotations.
 # Then everything is once again consistent with the non-vendored world.
 
 go mod edit -go=1.17
 go mod vendor
+
+grep '^## explicit; go 1.17$' vendor/modules.txt
 go build example.net/need117
+
+grep '^## explicit; go 1.13$' vendor/modules.txt
 ! go build example.net/bad114
 stderr '^vendor[/\\]example\.net[/\\]bad114[/\\]bad114.go:15:2: duplicate method Y$'
 
-
 -- go.mod --
 module example.net/m