back.rtlil: remove _SyncBuilder. NFC

Amaranth doesn't emit sync rules for a while since these are private
for the Yosys Verilog frontend.

Author: whitequark

amaranth/back/rtlil.py

@@ -215,9 +215,6 @@ def __exit__(self, *args):
     def case(self):
         return _CaseBuilder(self, indent=2)
 
-    def sync(self, kind, cond=None):
-        return _SyncBuilder(self, kind, cond)
-
 
 class _CaseBuilder(_ProxiedBuilder):
     def __init__(self, rtlil, indent):
@@ -264,26 +261,6 @@ def case(self, *values, attrs={}, src=""):
         return _CaseBuilder(self.rtlil, self.indent + 2)
 
 
-class _SyncBuilder(_ProxiedBuilder):
-    def __init__(self, rtlil, kind, cond):
-        self.rtlil = rtlil
-        self.kind  = kind
-        self.cond  = cond
-
-    def __enter__(self):
-        if self.cond is None:
-            self._append("    sync {}\n", self.kind)
-        else:
-            self._append("    sync {} {}\n", self.kind, self.cond)
-        return self
-
-    def __exit__(self, *args):
-        pass
-
-    def update(self, lhs, rhs):
-        self._append("      update {} {}\n", lhs, rhs)
-
-
 def _src(src_loc):
     if src_loc is None:
         return None