Remove unused task syscalls

[nroggeman-ledger]

Description

The goal of this PR is to:

• Remove the not-tested/not-used support of sub-tasks creation in Applications.
• Move private OS_Sched public syscalls to OS (os_sched_is_running(), os_sched_yield(), os_sched_switch(), os_sched_current_task())

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)
• Tests
• Documentation
• Other (for changes that might not fit in any category)

Breaking changes

Removed syscalls (but not used by any app)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.82%. Comparing base (144baa2) to head (eef2a42).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1424   +/-   ##
=======================================
  Coverage   91.82%   91.82%           
=======================================
  Files          37       37           
  Lines        4244     4244           
  Branches      521      521           
=======================================
  Hits         3897     3897           
  Misses        257      257           
  Partials       90       90           

Flag	Coverage Δ
unittests	91.82% <ø> (ø)

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