bdiff: drop duplicate definition of splitnewlines()

It was added in 29dd37a418aa (bdiff: write a native version of
splitnewlines, 2018-01-25).

Differential Revision: https://phab.mercurial-scm.org/D5618

mercurial/pure/bdiff.py

@@ -90,13 +90,3 b' def fixws(text, allws):'
         text = re.sub('[ \t\r]+', ' ', text)
         text = text.replace(' \n', '\n')
     return text
-
-def splitnewlines(text):
-    '''like str.splitlines, but only split on newlines.'''
-    lines = [l + '\n' for l in text.split('\n')]
-    if lines:
-        if lines[-1] == '\n':
-            lines.pop()
-        else:
-            lines[-1] = lines[-1][:-1]
-    return lines