Fixed DEPRECATION WARNING about deprecated sass @extend .a.b in application.scss

[qichunren]

And &.active { @extend .btn-default.active } is not need,
because it already extend .btn-default and will auto extend .btn-default.active

DEPRECATION WARNING on line 524 of /home/xxxx/homeland/app/assets/stylesheets/application.scss:
Extending a compound selector, .btn-default.active, is deprecated and will not be supported in a future release.
Consider "@extend .btn-default, .active" instead.
See sass/sass#1599 for details.

这个extend不需要声明了，前面已经extend .btn-default了，会自动extend .btn-default的.active,我在浏览器中前后对比了，发现删除这个，生成的css是没有变的，所以我的改动是可行的，并且消除了DEPRECATION WARNING

[codecov]

Codecov Report

Merging #1044 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1044   +/-   ##
=======================================
  Coverage   98.47%   98.47%           
=======================================
  Files         131      131           
  Lines        5890     5890           
=======================================
  Hits         5800     5800           
  Misses         90       90

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 96f1528...31a85ef. Read the comment docs.