Drop cross-compiling support to certain platforms which are unsupported by 'rust-embedded/cross'

Reference: cross-rs/cross#306 (comment)

Author: vishaltelangre

.travis.yml

@@ -24,7 +24,6 @@ matrix:
     - env: TARGET=mipsel-unknown-linux-gnu
     - env: TARGET=powerpc-unknown-linux-gnu
     - env: TARGET=powerpc64-unknown-linux-gnu
-    - env: TARGET=powerpc64le-unknown-linux-gnu
     - env: TARGET=s390x-unknown-linux-gnu DISABLE_TESTS=1
     - env: TARGET=x86_64-unknown-linux-gnu
     - env: TARGET=x86_64-unknown-linux-musl
@@ -36,8 +35,6 @@ matrix:
       os: osx
 
     # *BSD
-    - env: TARGET=i686-unknown-freebsd DISABLE_TESTS=1
-    - env: TARGET=x86_64-unknown-freebsd DISABLE_TESTS=1
     - env: TARGET=x86_64-unknown-netbsd DISABLE_TESTS=1
 
     # Testing other channels