Fix flight form & fall damage

[vtronko]

Fixes #16627 issue.
This handles all variations of that uninteded behavior, including druid flight form and .gm fly commands. Tested in-game, it works flawlessly.

[Shauren]

I'm liking this idea but I would like you to change two things

• Codestyle - don't wrap the if in one line and dont wrap single statements in brackets
• Use SetFallInformation(0, GetPositionZ()); instead of assigning m_lastFallZ directly (you aren't resetting fall time variable properly)

[vtronko]

@Shauren i was thinking about SetFallInformation(0, GetPositionZ()), but we already set fallTime to 0 in Unit::SetCanFly, so we will touch this variable two times.
But if you think it's better idea - here is updated diff.

[Shauren]

Unit::SetCanFly sets that value in movementInfo.fallTime field, SetFallInformation modifies m_lastFallTime (member of Player class)