base: remove env_deprecated from thread-start code

This patch replaces the internal use 'env_deprecated()' from the
implementation of the thread API in the base library. It also
replaces the global accessor 'main_thread_cap' by the explicit
propagation of the main-thread's capability to the single point of
use via a new 'init_thread_bootstap' function.

Issue #4784

repos/base/src/core/main.cc

@@ -90,9 +90,6 @@ Platform_generic &Core::platform() { return platform_specific(); }
 void Genode::init_platform() { core_env(); }
 
 
-Thread_capability Genode::main_thread_cap() { return Thread_capability(); }
-
-
 /**
  * Dummy implementation for core that has no parent to ask for resources
  */