revert #20719; relieve std/assertions of the sysFatal dep (#20743)

* Revert "make `system/fatal` importable (#20718)"

This reverts commit d735c447d3.

* relieve `std/assertions` of the sysFatal dep

tests/assert/tassert_c.nim

@@ -8,7 +8,7 @@ tassert_c.nim(35)        tassert_c
 tassert_c.nim(34)        foo
 assertions.nim(*)       failedAssertImpl
 assertions.nim(*)       raiseAssert
-fatal.nim(*)            sysFatal"""
+"""
 
 proc tmatch(x, p: string): bool =
   var i = 0

tests/errmsgs/t9768.nim

@@ -1,10 +1,10 @@
 discard """
-  errormsg: "unhandled exception:"
-  file: "system/fatal.nim"
+  errormsg: "unhandled exception: t9768.nim(24, 12) `a < 4`  [AssertionDefect]"
+  file: "std/assertions.nim"
   nimout: '''
 stack trace: (most recent call last)
-t9768.nim(28, 33)        main
-t9768.nim(23, 11)        foo1
+t9768.nim(29, 33)        main
+t9768.nim(24, 11)        foo1
 '''
 """
 
@@ -17,6 +17,7 @@ t9768.nim(23, 11)        foo1
 
 
 
+
 ## line 20
 
 proc foo1(a: int): auto =