Remove cmake code for unity builds

It serves little purpose because it basically duplicates cmake's
CMAKE_UNITY_BUILD switch and even causes confusion if they
are both set to different values. If I pass -DCMAKE_UNITY_BUILD=FALSE
to cmake >= 3.16.0 for example, I wouldn't expect a unity build but
that's what I would get.
Furthermore some parts are even discouraged by the official cmake
documentation. It states "Projects should not directly
set the UNITY_BUILD property or its associated CMAKE_UNITY_BUILD
variable to true" [1].
However, the documentation recommends to set the UNITY_BUILD to FALSE
for targets known to cause problems with unity builds. cmake < 3.16.0
seems to just ignore that (tested with 3.15.7), so no need for a
version check.

[1] https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.html
2 jobs for !54 with work/remove-cmake-unity-code in 16 minutes and 54 seconds
latest detached
Status Job ID Name Coverage
  Build
failed #97332
build_clazy_clang_tidy

00:04:19

passed #97331
linux kf5-qt5 qt5.15

00:12:35

1.0%
 
Name Stage Failure
failed
build_clazy_clang_tidy Build
make[2]: *** [src/shared/CMakeFiles/akonadi_shared.dir/build.make:121: src/shared/CMakeFiles/akonadi_shared.dir/akremotelog.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:2627: src/shared/CMakeFiles/akonadi_shared.dir/all] Error 2
make: *** [Makefile:160: all] Error 2
Uploading artifacts for failed job
Uploading artifacts...
WARNING: build/clang-tidy-report.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1