system.nim cleanup some exported constants which should never have be… (#17909)

* system.nim cleanup some exported constants which should never have been exported

lib/system.nim

@@ -537,7 +537,7 @@ const
 
 include "system/inclrtl"
 
-const NoFakeVars* = defined(nimscript) ## `true` if the backend doesn't support \
+const NoFakeVars = defined(nimscript) ## `true` if the backend doesn't support \
   ## "fake variables" like `var EBADF {.importc.}: cint`.
 
 const notJSnotNims = not defined(js) and not defined(nimscript)
@@ -1132,7 +1132,6 @@ const
 const
   hasThreadSupport = compileOption("threads") and not defined(nimscript)
   hasSharedHeap = defined(boehmgc) or defined(gogc) # don't share heaps; every thread has its own
-  nimEnableCovariance* = defined(nimEnableCovariance) # or true
 
 when hasThreadSupport and defined(tcc) and not compileOption("tlsEmulation"):
   # tcc doesn't support TLS
@@ -1920,24 +1919,7 @@ include "system/gc_interface"
 # we have to compute this here before turning it off in except.nim anyway ...
 const NimStackTrace = compileOption("stacktrace")
 
-template coroutinesSupportedPlatform(): bool =
-  when defined(sparc) or defined(ELATE) or defined(boehmgc) or defined(gogc) or
-    defined(nogc) or defined(gcRegions) or defined(gcMarkAndSweep):
-    false
-  else:
-    true
-
-when defined(nimCoroutines):
-  # Explicit opt-in.
-  when not coroutinesSupportedPlatform():
-    {.error: "Coroutines are not supported on this architecture and/or garbage collector.".}
-  const nimCoroutines* = true
-elif defined(noNimCoroutines):
-  # Explicit opt-out.
-  const nimCoroutines* = false
-else:
-  # Autodetect coroutine support.
-  const nimCoroutines* = false
+import system/coro_detection
 
 {.push checks: off.}
 # obviously we cannot generate checking operations here :-)