QDoc: Stop defining QDOC2_COMPAT

At the build system level, QDoc continues to define
`QDOC2_COMPAT`. This define is unused, and has been so for several
years. Commit `d259c70d47d0dbf2fb49b6fdb52b1283e8a69a41` removed the
definition from the qmake configuration; however, the port to CMake
overlapped this and accidentally re-introduced the definition with
commit `9d5887fbc6fe0b20c4faf7eb42f1b71130e95120`.

Commit `a40749199be276aa89fcb4c171c4943a581d91de` removed code that
required the definition of `QDOC2_COMPAT`. Since that commit, the
definition has been unused.

This change seeks to remove this define for good, as it does
absolutely nothing, and hasn't for many years.

Pick-to: 6.8
Change-Id: I00c618a805faf7bac5cc52d0ce6d1c927865ea44
Reviewed-by: Topi Reiniö <[email protected]>

Author: paulwicking

src/qdoc/qdoc/CMakeLists.txt

@@ -96,7 +96,6 @@ qt_internal_add_tool(${target_name}
         # To provide the ability to workaround version-specific Clang issues.
         # A re-export of (LLVM|CLANG)_VERSION_MAJOR done in WrapLibClang.cmake
         LIBCLANG_VERSION_MAJOR=${QT_LIB_CLANG_VERSION_MAJOR}
-        QDOC2_COMPAT
 )
 qt_internal_return_unless_building_tools()