Pragma once instead of header guards

[Konard]

Support for header guards is dropped.
The only way we prevent repeated inclusion of headers is by adding #pragma once directive.

Pros:

• Less code to write and maintain

Cons:

• Some old versions of compilers may not be supported

This design decision should be applied to all C++ code in all LinksPlatform's repositories, unless it is impossible to do so. The change should be applied by lazy refactoring or automation.