0.15: Do not install toml

[ax3l]

When doing the default super-build, the toml sub-project should not be installed.
It's a private dependency.

Follow-up to #1148 #1146.

Similar to:

• NLohmann-JSON: 3.7.0+ #556 / CMake: Optional Install if Embedded nlohmann/json#1330
• https://stackoverflow.com/questions/35344132/cpack-exclude-install-commands-from-subdirectory-googletest-directory/51746390#51746390

[ax3l]

X-ref: ToruNiina/toml11#185

[franzpoeschel]

X-ref: ToruNiina/toml11#185

Should I close https://github.com/openPMD/openPMD-api/pull/1210/files then and we go with your solution as soon as it's merged?

[ax3l]

We can keep that PR open in case toml11 merges our PR too slowly (aka before we want to cut 0.15.0).

[ax3l]

Fixed via #1227