Remove 8.0-rc/alpine3.12/zts variant

See #1028 (comment) for details -- Someone™ needs to dig into the `pdo_mysql` build issue for this to return:

> `PHP Warning: PHP Startup: Unable to load dynamic library 'pdo_mysql.so' (tried: /usr/local/lib/php/extensions/no-debug-zts-20190128/pdo_mysql.so (Error relocating /usr/local/lib/php/extensions/no-debug-zts-20190128/pdo_mysql.so: _tsrm_ls_cache: initial-exec TLS resolves to dynamic definition in /usr/local/lib/php/extensions/no-debug-zts-20190128/pdo_mysql.so)`

Author: tianon

.travis.yml

@@ -8,7 +8,6 @@ env:
   - VERSION=8.0-rc VARIANT=buster/zts
   - VERSION=8.0-rc VARIANT=alpine3.12/cli
   - VERSION=8.0-rc VARIANT=alpine3.12/fpm
-  - VERSION=8.0-rc VARIANT=alpine3.12/zts
   - VERSION=7.4 VARIANT=buster/cli
   - VERSION=7.4 VARIANT=buster/apache
   - VERSION=7.4 VARIANT=buster/fpm