travis: remove an obsolete pre-build hack

We made a mistake when tagging 1.7.5, which confused git describe.
Now we have added more tags and the hack with removal can be deleted.

.travis.yml

@@ -72,7 +72,6 @@ matrix:
         os: osx
 
 script:
-  - git tag -d 1.7.5
   - make -f .travis.mk ${TARGET}
 
 before_deploy: