feature symfony#16637 [3.0] Removed unneeded polyfill (GrahamCampbell)

This PR was merged into the 3.0-dev branch.

Discussion
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[3.0] Removed unneeded polyfill

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

Surely we don't need these on 3.x?

Commits
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42c9a48 Removed unneeded polyfill from the master

Author: Tobion

composer.json

@@ -22,8 +22,6 @@
         "psr/log": "~1.0",
         "symfony/polyfill-intl-icu": "~1.0",
         "symfony/polyfill-mbstring": "~1.0",
-        "symfony/polyfill-php54": "~1.0",
-        "symfony/polyfill-php55": "~1.0",
         "symfony/polyfill-php56": "~1.0",
         "symfony/polyfill-php70": "~1.0",
         "symfony/polyfill-util": "~1.0"