Remove USE_BAZEL and its dead code

This environment variable has been out of use for some time. It used to
control Bazel-as-executor, which has been removed from Bazel for several
releases.

Bug: 243077098
Test: Treehugger
Change-Id: I4edfb5f3574c369a16547ea06ba780293c37ecb3

ui/build/build.go

@@ -104,10 +104,9 @@ const (
 	// Whether to run ninja on the combined ninja.
 	RunNinja = 1 << iota
 	// Whether to run bazel on the combined ninja.
-	RunBazel        = 1 << iota
-	RunBuildTests   = 1 << iota
-	RunAll          = RunProductConfig | RunSoong | RunKati | RunKatiNinja | RunNinja
-	RunAllWithBazel = RunProductConfig | RunSoong | RunKati | RunKatiNinja | RunBazel
+	RunBazel      = 1 << iota
+	RunBuildTests = 1 << iota
+	RunAll        = RunProductConfig | RunSoong | RunKati | RunKatiNinja | RunNinja
 )
 
 // checkBazelMode fails the build if there are conflicting arguments for which bazel
@@ -256,9 +255,6 @@ func Build(ctx Context, config Config) {
 	SetupPath(ctx, config)
 
 	what := RunAll
-	if config.UseBazel() {
-		what = RunAllWithBazel
-	}
 	if config.Checkbuild() {
 		what |= RunBuildTests
 	}