travis: remove platform option arm64

[1] https://github.com/asciimoo/searx/pull/1910#issuecomment-607244871

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

.travis.yml

@@ -1,6 +1,5 @@
 os: linux
 dist: bionic
-arch: arm64
 language: python
 cache:
   - directories: