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/linker.go b/cc/linker.go
index f346584..4e9404c 100644
--- a/cc/linker.go
+++ b/cc/linker.go
@@ -389,9 +389,7 @@
 	}
 
 	deps.SystemSharedLibs = linker.Properties.System_shared_libs
-	// In Bazel conversion mode, variations have not been specified, so SystemSharedLibs may
-	// inaccuarately appear unset, which can cause issues with circular dependencies.
-	if deps.SystemSharedLibs == nil && !ctx.BazelConversionMode() {
+	if deps.SystemSharedLibs == nil {
 		// Provide a default system_shared_libs if it is unspecified. Note: If an
 		// empty list [] is specified, it implies that the module declines the
 		// default system_shared_libs.