travis: Remove global compiler variable

The global variable is not necessary since
the variable is defined in the matrix.

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

.travis.yml

@@ -45,9 +45,6 @@ notifications:
     on_failure: always  # options: [always|never|change] default: always
     on_start: false     # default: false
 
-compiler:
-  - gcc
-
 env:
   global:
     # The next declaration is the encrypted COVERITY_SCAN_TOKEN, created