Remove "go env" from gobuild linter

I see no reason to do this? It is just setting the environment to what
it already is?

It was originally added in #297, but that entire PR is not a great idea
in the first place; that PR (together with #270) tried to make the Go do
non-standard and non-supported stuff like compiling packages outside of
GOPATH.

That's not something that works well (I tried), so was eventually
removed in #465, but these "go env" calls remained, for no reason in
particular, as far as I can think of.

This will improve on #1834; you will now no longer get a confusing error
(but still won't get a meaningful error; need to think how to do that).

test/command_callback/test_gobuild_command_callback.vader

@@ -1,36 +1,19 @@
 Before:
   call ale#assert#SetUpLinterTest('go', 'gobuild')
 
-  let g:env_prefix = has('win32')
-  \ ? 'set GOPATH=' . ale#Escape('/foo/bar') . ' && '
-  \ : 'GOPATH=' . ale#Escape('/foo/bar') . ' '
-  call ale_linters#go#gobuild#ResetEnv()
-
   WithChainResults ['/foo/bar', '/foo/baz']
 
 After:
-  unlet! g:env_prefix
   call ale#assert#TearDownLinterTest()
 
 Execute(The default commands should be correct):
-  AssertLinter 'go', [
-  \ 'go env GOPATH GOROOT',
-  \ g:env_prefix . 'cd ' . ale#Escape(expand('%:p:h')) . ' && '
+  AssertLinter 'go',
+  \ 'cd ' . ale#Escape(expand('%:p:h')) . ' && '
   \   . 'go test -c -o /dev/null ./'
-  \]
-
-  " We shouldn't run `go env` many times after we've got it.
-  AssertLinter 'go', [
-  \ '',
-  \ g:env_prefix . 'cd ' . ale#Escape(expand('%:p:h')) . ' && '
-  \   . 'go test -c -o /dev/null ./'
-  \]
 
 Execute(Extra options should be supported):
   let g:ale_go_gobuild_options = '--foo-bar'
 
-  AssertLinter 'go', [
-  \ 'go env GOPATH GOROOT',
-  \ g:env_prefix . 'cd ' . ale#Escape(expand('%:p:h')) . ' && '
+  AssertLinter 'go',
+  \ 'cd ' . ale#Escape(expand('%:p:h')) . ' && '
   \   . 'go test --foo-bar -c -o /dev/null ./'
-  \]