Remove the Santa kernel extension.

[tnek]

This PR removes:

• All of the code in Source/santa_driver containing the kernel extension
• The SNTDriverManager event provider
• All workflows in our CI related to testing if the driver builds
• Installation of the driver in install.sh. Note that code uninstalling existing instances of the driver is still intentionally kept present.
• Kernel extension-specific build rules
• Renames SNTKernelCommon to SNTCommon
• Driver version output from santactl version
• The [SNTConfigurator EnableSystemExtension] configuration key

[tnek]

Caught a few other references to SNTDriverManager to remove that was breaking tests. PTAL, this required rewriting parts of a few unit tests to mock an abstract implementation of the EventProvider protocol instead of the more concrete SNTDriverManager class.

[pmarkowsky]

This is the bulk of the work for #738.

Still need to remove the enums.