Remove custom bootloader platform

A dedicated boot_stage2 platform introduces a lot of complexity that
needs to be more thought-through.

bazel/platform/BUILD.bazel

diff --git a/bazel/platform/BUILD.bazel b/bazel/platform/BUILD.bazel
index ca9b829..467c0b1 100644
--- a/bazel/platform/BUILD.bazel
+++ b/bazel/platform/BUILD.bazel
@@ -7,9 +7,3 @@
         "@platforms//cpu:armv6-m",  # This is just FYI.
     ],
 )
-
-# For now, the bootloader is effectively the same platform as the rp2040.
-platform(
-    name = "rp2040_bootloader",
-    parents = [":rp2040"],
-)

bazel/util/transition.bzl

diff --git a/bazel/util/transition.bzl b/bazel/util/transition.bzl
index ee5947d..c768c0a 100644
--- a/bazel/util/transition.bzl
+++ b/bazel/util/transition.bzl
@@ -65,10 +65,8 @@
 rp2040_bootloader_binary = declare_transtion(
     attrs = {
         "_malloc": attr.label(default = "//src/rp2_common/boot_stage2:no_malloc"),
-        "_bootloader_platform": attr.label(default = "//bazel/platform:rp2040_bootloader"),
     },
     flag_overrides = {
-        "//command_line_option:platforms": "_bootloader_platform",
         # We don't want --custom_malloc to ever apply to the bootloader, so
         # always explicitly override it here.
         "//command_line_option:custom_malloc": "_malloc",