Clean up some bp2build technical debt

Previously we ran mutators in bp2build mode to add dependencies, now we
look up modules by name directly. Remove workarounds to allow bp2build
mode to not fail when adding/handling dependencies.

Test: m bp2build
Change-Id: Ibf6fd905150cac306e5c395902ef28f609f4df2a
diff --git a/cc/config/toolchain.go b/cc/config/toolchain.go
index 7175fdc..253bb06 100644
--- a/cc/config/toolchain.go
+++ b/cc/config/toolchain.go
@@ -36,19 +36,10 @@
 	Arch() android.Arch
 }
 
-type conversionContext interface {
-	BazelConversionMode() bool
-}
-
 func FindToolchainWithContext(ctx toolchainContext) Toolchain {
 	t, err := findToolchain(ctx.Os(), ctx.Arch())
 	if err != nil {
-		if c, ok := ctx.(conversionContext); ok && c.BazelConversionMode() {
-			// TODO(b/179123288): determine conversion for toolchain
-			return &toolchainX86_64{}
-		} else {
-			panic(err)
-		}
+		panic(err)
 	}
 	return t
 }