Makefile: don't set GOROOT

This *should* not be needed. Using the right `go` binary should do the
job. It might break if users have explicitly set GOROOT (in which case
they probably need to fix that).

This avoids a subprocess call inside `make`.

GNUmakefile

@@ -34,7 +34,6 @@ LLVM_NM ?= $(call findLLVMTool,llvm-nm)
 
 # Go binary and GOROOT to select
 GO ?= go
-export GOROOT = $(shell $(GO) env GOROOT)
 
 # Flags to pass to go test.
 GOTESTFLAGS ?=