Cleaned up sequence.py. Closes #1511.

[trevorbaca]

REMOVED abjad.sequence.map(). Use a list comprehension instead.

REMOVED abjad.sequence.sum(). Use built-in sum() instead.

REMOVED abjad.sequence.sort(). Use build-in sorted() instead.

REMOVED abjad.sequence.retain(). Use abjad.sequence.retain_pattern() instead.

CHANGED abjad.sequence.remove() input to pattern:

 OLD: abjad.sequence.remove(..., indices=[0, 1], period=5)
 NEW: abjad.sequence.remove(..., abjad.index([0, 1], 5))