Improve removeListener performance

[lpinca]

This sligtly improves the removeListener performance when there is only one listener.

Result of once.js benchmark:

log:      Finished benchmarking: "EventEmitter3@1.2.0"
metric:   Count (51502), Cycles (5), Elapsed (5.592), Hz (913621.98)
log:      Finished benchmarking: "EventEmitter3(master)"
metric:   Count (55202), Cycles (4), Elapsed (5.583), Hz (978589.2)
info:     Benchmark: "EventEmitter3(master)" is the fastest.

Result of listening.js benchmark:

log:      Finished benchmarking: "EventEmitter3@1.2.0"
metric:   Count (58987), Cycles (5), Elapsed (5.745), Hz (1090032.06)
log:      Finished benchmarking: "EventEmitter3(master)"
metric:   Count (64261), Cycles (9), Elapsed (5.706), Hz (1181078.63)
info:     Benchmark: "EventEmitter3(master)" is the fastest.

[coveralls]

Coverage remained the same at 100.0% when pulling e0e422c on optimization into 009d550 on master.

[3rd-Eden]

LGTM