Get rid of ureact_amalgamated.hpp

[YarikTH]

Description

ureact/single_include/ureact/ureact_amalgamated.hpp is currently used only for usage in godbolt. It is not installed as part of the library package, it is not provided as part of CMake target, it just exists. And multiplies each library change in two (at least at release time).

So, now, when ureact is added on godbolt, its time to get rid of it and its usage. To do so, need to remove the file, make sure make amalgamate is not in the documentation somewhere and check and fix existing godbolt links which use ureact_amalgamated.hpp.

There is no need in getting rid of make amalgamate itself. godbolt clones main and dev branches nightly, so if something needs testing now, it can be used to create ureact_amalgamated.hpp in the temporary branch.

[YarikTH]

Done