Remove obsolete BuildTools folder

The BuildTools folder contained utilities from the era of targeting
multiple .NET Frameworks (2.0-4.5). These tools are no longer necessary
as the project now uses modern SDK-style projects targeting .NET
Framework 4.6.2.

Removed tools:
- VersionUpdater: Updated .NET Framework versions in project files
- SourceUpdater: Bulk source file updates (referenced SVN)
- SearchAndReplace: Text search and replace tool
- ReferenceAnalyzer: Project reference analyzer

These tools were last modified in 2013-2016 and are not referenced in
any build scripts, CI/CD configuration, or documentation. Modern
alternatives are built into Visual Studio and MSBuild.

Fixes #446

Co-Authored-By: Claude Code Sonnet 4.5

Author: jozefizso