Revert "add unsetenv, setenv, clearenv to os module (#583)"

This reverts commit 6cb5eee1b2.

os/os.v

@@ -316,24 +316,6 @@ pub fn getenv(key string) string {
 	return tos2(s)
 }
 
-
-pub fn setenv(name string, value string, overwrite bool) int {
-  return C.setenv(name.cstr(), value.cstr(), overwrite)
-}
-
-pub fn unsetenv(name string) int {
-  return C.unsetenv(name.cstr())
-}
-
-pub fn clearenv() {
-  C.clearenv()
-}
-
-fn exit(code int) {
-	C.exit(code)
-}
-
-
 // `file_exists` returns true if `path` exists.
 pub fn file_exists(path string) bool {
 	res := false

os/os_test.v

@@ -1,26 +0,0 @@
-import os
-
-fn test_setenv() {
-  os.setenv('foo', 'bar', true)
-  assert os.getenv('foo') == 'bar'
-  
-  // `setenv` should not set if `overwrite` is false
-  os.setenv('foo', 'bar2', false)
-  assert os.getenv('foo') == 'bar'
-  
-  // `setenv` should overwrite if `overwrite` is true
-  os.setenv('foo', 'bar2', true)
-  assert os.getenv('foo') == 'bar2'
-}
-
-fn test_unsetenv() {
-  os.setenv('foo', 'bar', true)
-  os.unsetenv('foo')
-  assert os.getenv('foo') == ''
-}
-
-fn test_clearenv() {
-  os.setenv('foo', 'bar', true)
-  os.clearenv()
-  assert os.getenv('foo') == ''
-}