Remove the instruction for Ruby version for Ruby 1.9 era

Debian 7 (wheezy) included ruby1.8 package
https://wiki.debian.org/Ruby

This instruction was introduced in d58aca06 on 2013-05-24.
https://github.com/gitlabhq/gitlabhq/commit/d58aca0693d28e17633fb0d2825f1cddfd4f0748

Changelog: fixed
Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

doc/install/installation.md

@@ -221,12 +221,6 @@ below to use a system Ruby.
 Linux distributions generally have older versions of Ruby available, so these
 instructions are designed to install Ruby from the official source code.
 
-Remove the old Ruby 1.8 if present:
-
-```shell
-sudo apt-get remove ruby1.8
-```
-
 Download Ruby and compile it:
 
 ```shell