SCons: Move platform logo/run icon to export folder

Follow-up to #75932.
Since these icons are only used by the export plugin, it makes sense to
move them and generate the headers there.

The whole `detect.is_active()` logic seems to be a leftover from before
times, as far back as 1.0-stable it already wasn't used for anything.

So I'm removing it and moving the export icon generation to
`platform_methods`, where it makes more sense.

methods.py

@@ -559,33 +559,6 @@ def use_windows_spawn_fix(self, platform=None):
     self["SPAWN"] = mySpawn
 
 
-def save_active_platforms(apnames, ap):
-    for x in ap:
-        svg_names = []
-        if os.path.isfile(x + "/logo.svg"):
-            svg_names.append("logo")
-        if os.path.isfile(x + "/run_icon.svg"):
-            svg_names.append("run_icon")
-
-        for name in svg_names:
-            svgf = open(x + "/" + name + ".svg", "rb")
-            b = svgf.read(1)
-            svg_str = " /* AUTOGENERATED FILE, DO NOT EDIT */ \n"
-            svg_str += " static const char *_" + x[9:] + "_" + name + '_svg = "'
-            while len(b) == 1:
-                svg_str += "\\" + hex(ord(b))[1:]
-                b = svgf.read(1)
-
-            svg_str += '";\n'
-
-            svgf.close()
-
-            # NOTE: It is safe to generate this file here, since this is still executed serially
-            wf = x + "/" + name + "_svg.gen.h"
-            with open(wf, "w") as svgw:
-                svgw.write(svg_str)
-
-
 def no_verbose(sys, env):
     colors = {}