Remove the Platform WiX variable

[volks73]

As discussed in #113, the Platform WiX variable is redundant to the sys.BUILDARCH WiX variable. It is no longer needed in the embedded WXS template. Note, this is the WiX Variable, not to be confused with the Platform attribute of the Product tag of the WXS schema, which is also deprecated.

Some users may have WXS files that depend on the Platform WiX variable. While I want to avoid making more work for users, we cannot be prevented from evolving and fixing inconsistencies that would add to the technical debt of this project. Thus, this issue is to notify users that the variable will eventually be removed.

Note, it is possible to use cargo install --version 0.3.1 cargo-wix to install a specific version of this subcommand that will continue to provide the Platform Wix variable.

The documentation will be updated to indicate the variable is deprecated and its usage should be avoided in new projects, along with a migration example using the sys.BUILDARCH WiX variable.

[volks73]

The Platform WiX variable still exists and is available even though #126 and #127 have been resolved.