CodeBlocks/MinGw: remove -march=native

1. I discovered that this removes the duplicate section linker warnings...
2. The option causes binaries to be incompatible with different hardware.

projectfiles/CodeBlocks/README.txt

@@ -52,10 +52,3 @@ when finished building.
 10. To be able to execute the program, copy all *.dll files from your
 installation folder of the most recent official windows wesnoth development
 release into wesnoth_root/.
-
-note:
-Executables created by this built system cannot easily be shared with
-other computers due to the currently set -march=native option.
-If you happen to meet such a problem, try removing it. (right-click
-projects wesnoth and wesnothd>project settings>project's build options
->other options)

projectfiles/CodeBlocks/wesnoth.cbp

@@ -31,7 +31,6 @@
 		<Compiler>
 			<Add option="-Wall" />
 			<Add option="-mthreads" />
-			<Add option="-march=native" />
 			<Add option="-fexceptions" />
 			<Add option="-fdiagnostics-show-option" />
 			<Add option="-fopenmp" />

projectfiles/CodeBlocks/wesnothd.cbp

@@ -31,7 +31,6 @@
 			<Add option="-Wall" />
 			<Add option="-mthreads" />
 			<Add option="-fexceptions" />
-			<Add option="-march=native" />
 			<Add option="-fdiagnostics-show-option" />
 			<Add option="-fopenmp" />
 			<Add option="-DHAVE_PYTHON" />