go.mod: remove spurious x/tools replace directive

Remove bogus x/tools replace directive that was added in CL 649317,
because I ran the go mod edit command from the wrong directory.

Apparently, it did not matter for the CL, because gopls is unaffected by
replace directives in x/tools. Furthermore, the resulting prerelease
still installs successfully.

Change-Id: I731764abd522a515bacab51bcbb3de830378ec02
Reviewed-on: https://go-review.googlesource.com/c/tools/+/649735
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Robert Findley <[email protected]>

Author: findleyr

go.mod

@@ -12,5 +12,3 @@ require (
 )
 
 require golang.org/x/sys v0.30.0 // indirect
-
-replace golang.org/x/tools => ../