Stop setting TERM in history test

Because TERM was set to something other than 'dumb', we were subject to
syntax highlighting and other interactive features that would affect the
output. In practice we were getting lucky timing-wise, but with upcoming
interactive changes syntax highlighting started to fail this test.

tests/pexpects/history.py

@@ -15,7 +15,6 @@ from pexpect_helper import SpawnedProc
 import os
 
 os.environ["PAGER"] = "cat"
-os.environ["TERM"] = "xterm"
 
 sp = SpawnedProc(env=os.environ.copy())