feature #16381 remove polyfills for unsupported php versions (Tobion)

This PR was merged into the 3.0-dev branch.

Discussion
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remove polyfills for unsupported php versions

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

Remove obsolete polyfills in master as introduced in #16317

Commits
-------

78512cc remove polyfills for unsupported php versions

Author: Tobion

composer.json

@@ -16,8 +16,7 @@
         }
     ],
     "require": {
-        "php": ">=5.5.9",
-        "symfony/polyfill-php55": "~1.0"
+        "php": ">=5.5.9"
     },
     "require-dev": {
         "symfony/yaml": "~2.8|~3.0",