Replace .border-radius mixin with border-radius css in brackets_patterns_override.less

[ficristo]

Description

In src/styles/brackets_patterns_override.less there are some mentions of .border-radius mixin.
These can safely replaced with the corresponding css border-radius.
So .border-radius(0 0 3px 3px); will be border-radius: 0 0 3px 3px;

There are some other mentions in the tree, but they are in bootstrap files or in files where it is necessary, so there should be left untouched.

[parthsharma2]

Hi, I would like to take this issue

[parthsharma2]

Made a pull request to fix this issue. Checkout #13235