Remove gdbm related code from Travis CI

The related extensions have been removed.

Related: https://github.com/ruby/ruby/pull/4619

.travis.yml

@@ -64,7 +64,6 @@ env:
         gcc-10
         g++-10
         libffi-dev
-        libgdbm-dev
         libncurses-dev
         libncursesw5-dev
         libreadline-dev
@@ -110,7 +109,6 @@ env:
         libc6:armhf
         libstdc++-10-dev:armhf
         libffi-dev:armhf
-        libgdbm-dev:armhf
         libncurses-dev:armhf
         libncursesw5-dev:armhf
         libreadline-dev:armhf