Remove option to mark individual functions as extern C

src/pystencilssfg/composer/basic_composer.py

@@ -384,7 +384,7 @@ class SfgBasicComposer(SfgIComposer):
         if self._ctx.impl_file is None:
             seq.inline()
         if self._ctx.c_interfacing:
-            seq.externC()
+            seq._externC = True
 
         return seq
 
@@ -701,11 +701,6 @@ class SfgFunctionSequencerBase:
         self._inline = True
         return self
 
-    def externC(self):
-        """Mark this function as ``extern "C"``."""
-        self._externC = True
-        return self
-
     def constexpr(self):
         """Mark this function as ``constexpr``."""
         self._constexpr = True