Remove unused build output buffer (#4786)

* Remove unused build output buffer

* Update proc.py

* Update proc.py

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Co-authored-by: Ivan Kravets <me@ikravets.com>

platformio/proc.py

@@ -69,7 +69,7 @@ class BuildAsyncPipe(AsyncPipeBase):
         print_immediately = False
 
         for char in iter(lambda: self._pipe_reader.read(1), ""):
-            self._buffer += char
+            # self._buffer += char
 
             if line and char.strip() and line[-3:] == (char * 3):
                 print_immediately = True