[C] Strip OSX special characters from sign text. Fixes BUKKIT-5102

**The Issue**
On Mac OSX clients, pressing the up/down arrow keys inserts special characters into sign text (while being edited, of course). This causes an issue with plugins attempting to parse the text, as they aren't expecting these characters, and may not match where it should. These characters are invisible to the client.

**Justification for this PR**
This PR fixes the issue by removing the special characters when the sign is created.

**PR Breakdown**
When a CraftSign is created, it iterates through the lines and replaces the characters (\uF700 and \uF701) with "".

**Testing results and materials**
I tested this by attempting to create an Essentials sign on both the latest version of Craftbukkit, and running this one, pressing the down arrow before closing the editor. Without this commit, the sign isn't recognized by the plugin and acts as a normal sign. With this commit, the special characters are skipped and the plugin behaves normally.

**Relevant PRs**
None

**JIRA Ticket**
BUKKIT-5102 - https://bukkit.atlassian.net/browse/BUKKIT-5102