minor #14626 [travis] Don't use the cache (nicolas-grekas)

This PR was merged into the 2.3 branch.

Discussion
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[travis] Don't use the cache

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

The composer cache breaks per components builds (deps=low/high)

Commits
-------

103c0df [travis] Don't use the cache

.travis.yml

@@ -2,10 +2,6 @@ language: php
 
 sudo: false
 
-cache:
-  directories:
-    - $HOME/.composer/cache
-
 addons:
   apt_packages:
     - parallel