Remove unnecessary null pointer checks

[elfring]

An extra null pointer check is not needed in functions like the following.

• Landusemap
• MovableText
• RoR::Terrain::dispose

[elfring]

:thought_balloon: Would you become interested to use a development tool like “clang-tidy” for corresponding source code adjustments?

[ohlidalp]

If you set up clang-tidy with this single rule only and create a pull request with it, I'll consider. I'm not a fan of major tool-based reformatting.