remove tools/Prepare-Release.ps1

[mast-eu]

Most of he original functionality provided by script tools/Prepare-Release.ps1 has been removed in PR #18.
This script is used only for CI builds and called from here:

gitextensions.pluginmanager/appveyor.yml

Lines 41 to 42 in 6b1c098

	build_script:
	- ps: .\tools\Prepare-Release.ps1

The remaining functionality of Prepare-Release.ps1 could be entirely moved into appveyor.yml. Afterwards we can fully remove the PowerShell script.