Fix minor error at ls color on common-aliases plugin

Removed ```alias ls='ls --color' #I like color``` on plugins/common-aliases.zsh

That line on OS X would cause:

```
ls: illegal option -- -
usage: ls [-ABCFGHLOPRSTUWabcdefghiklmnopqrstuwx1] [file ...]
```

And it's unnecessary since lib/theme-and-appearence.zsh already does:

```
# Enable ls colors
if [ "$DISABLE_LS_COLORS" != "true" ]
then
  # Find the option for using colors in ls, depending on the version: Linux or BSD
  if [[ "$(uname -s)" == "NetBSD" ]]; then
    # On NetBSD, test if "gls" (GNU ls) is installed (this one supports colors); 
    # otherwise, leave ls as is, because NetBSD's ls doesn't support -G
    gls --color -d . &>/dev/null 2>&1 && alias ls='gls --color=tty'
  elif [[ "$(uname -s)" == "OpenBSD" ]]; then
    # On OpenBSD, test if "colorls" is installed (this one supports colors);
    # otherwise, leave ls as is, because OpenBSD's ls doesn't support -G
    colorls -G -d . &>/dev/null 2>&1 && alias ls='colorls -G'
  else
    ls --color -d . &>/dev/null 2>&1 && alias ls='ls --color=tty' || alias ls='ls -G'
  fi
fi
```

Which maps correctly color flags on different OS's.