Remove unused `pytester.getdecoded` (#6805)

Last usage was removed in 22dc47d9f.

src/_pytest/pytester.py

@@ -25,7 +25,6 @@ import py
 
 import pytest
 from _pytest._code import Source
-from _pytest._io.saferepr import saferepr
 from _pytest.capture import MultiCapture
 from _pytest.capture import SysCapture
 from _pytest.compat import TYPE_CHECKING
@@ -1283,15 +1282,6 @@ class Testdir:
         return child
 
 
-def getdecoded(out):
-    try:
-        return out.decode("utf-8")
-    except UnicodeDecodeError:
-        return "INTERNAL not-utf8-decodeable, truncated string:\n{}".format(
-            saferepr(out)
-        )
-
-
 class LineComp:
     def __init__(self):
         self.stringio = StringIO()