FindTclsh: Drop Cygwin-specific behavior and use POSIX code path

Based on downstream patch from Cygwin package for CMake by Marco Atzeri:

* https://github.com/matzeri/cygwin-pkg/blob/64864eb8f0d635ea44226e4b9a41a7ca59a1c7dd/cmake/3.17.3-findtclsh.patch

Modules/FindTclsh.cmake

@@ -15,15 +15,8 @@ library is.  This code sets the following variables:
 
   TCLSH_FOUND = TRUE if tclsh has been found
   TCL_TCLSH = the path to the tclsh executable
-
-In cygwin, look for the cygwin version first.  Don't look for it later
-to avoid finding the cygwin version on a Win32 build.
 #]=======================================================================]
 
-if(CYGWIN)
-  find_program(TCL_TCLSH NAMES cygtclsh83 cygtclsh80)
-endif()
-
 get_filename_component(TK_WISH_PATH "${TK_WISH}" PATH)
 get_filename_component(TK_WISH_PATH_PARENT "${TK_WISH_PATH}" PATH)
 string(REGEX REPLACE