Simplify some code in ScriptTask::load.

[Ms2ger]

No description provided.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3550

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[jdm]

r+

[bors-servo]

saw approval from jdm
at Ms2ger@10bda1a

[bors-servo]

merging Ms2ger/servo/load-simplify = 10bda1a into auto

[bors-servo]

Ms2ger/servo/load-simplify = 10bda1a merged ok, testing candidate = b9edc22

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/375
success: http://build.servo.org/builders/linux2/builds/374
success: http://build.servo.org/builders/android/builds/410
success: http://build.servo.org/builders/mac1/builds/591
success: http://build.servo.org/builders/mac2/builds/592

[bors-servo]

fast-forwarding master to auto = b9edc22