Revert "equiv_opt to call async2sync when not -multiclock like SymbiYosys"

This reverts commit a39505e329cc05dbd4ad624a1cf0f6caf664fd9a.

passes/equiv/equiv_opt.cc

diff --git a/passes/equiv/equiv_opt.cc b/passes/equiv/equiv_opt.cc
index 9fe3bbd..d4c7f79 100644
--- a/passes/equiv/equiv_opt.cc
+++ b/passes/equiv/equiv_opt.cc
@@ -156,8 +156,6 @@
 		if (check_label("prove")) {
 			if (multiclock || help_mode)
 				run("clk2fflogic", "(only with -multiclock)");
-			else
-				run("async2sync", "(only without -multiclock)");
 			run("equiv_make gold gate equiv");
 			if (help_mode)
 				run("equiv_induct [-undef] equiv");