support/scripts/gen-bootlin-toolchains: drop "mips64" architecture variant

These toolchains have been marked obsolete by toolchains.bootlin.com
as they are replaced by the mips64-n32 toolchains. See commit:

  https://github.com/bootlin/toolchains-builder/commit/121e78806b4968858f22ebea317767883752a456

Will allow to fix:

  http://autobuild.buildroot.net/results/d13b35ba5a0f68f72e6592bdd9218b625a3c6554/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit db37e75aa3e9adf4cf2745682a5d29d246d88880)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/scripts/gen-bootlin-toolchains

@@ -99,11 +99,6 @@ arches = {
         'conditions': ['BR2_mipsel', 'BR2_mips_32r6', '!BR2_MIPS_SOFT_FLOAT'],
         'prefix': 'mipsel',
     },
-    'mips64': {
-        # Not sure it could be used by other mips64 variants?
-        'conditions': ['BR2_mips64', 'BR2_mips_64', '!BR2_MIPS_SOFT_FLOAT'],
-        'prefix': 'mips64',
-    },
     'mips64-n32': {
         # Not sure it could be used by other mips64 variants?
         'conditions': ['BR2_mips64', 'BR2_mips_64', 'BR2_MIPS_NABI32', '!BR2_MIPS_SOFT_FLOAT'],