Remove buggy line in install.sh causing "all" option to break

Remove buggy line in install.sh causing "all" option to break

When selecting all, I get the following in my plugins/enabled directory

$ cd ~/.bash_it/plugins/enabled && ls -l
<snip fileinfo>          * -> ~/.bash_it/plugins/[^_]available/*

In other words, the regexp is not being expanded, and I don't think bash
has ever had this capability (I am running 4.2.24) . Looking at the commit
24431627ab24c1c97bf3fb5796037e198f465e25, this line was added so as to disable
some plugins starting with a "_". In this circumstance, this line is
wrong anyway as it skips the directory "_available", rather than
available/_whatever.

This commit aims to fix this to the installer's intended purpose.
It also does a sanity check that no file exists already in enabled and
skips otherwise. ln -s does the right thing however more human error
message might be more desirable.