build_config: drop the host build from the IntelEdison config - #7247

build_config/IntelEdison.rb opened an anonymous `MRuby::Build` ahead of
its `CrossBuild`, and an unnamed build is called 'host'
(lib/mruby/build.rb:114).  The build tree is keyed by that name (:127),
so MRUBY_CONFIG=IntelEdison aimed a build carrying `MRB_USE_READLINE`
and mruby-hs-regexp at build/host, on top of whatever a default build
had left there.

The block dates from a27426a (2016), when a cross build took its mrbc
from `MRuby.targets['host']` and nowhere else, so every cross config
declared a host build to have one.  1520c97 (2020) made that
unnecessary and deleted the block from the eight other cross configs of
the time; this one, the only one with a comment header above the block
rather than the block at line 1, kept it.  Nothing here has needed it
since: the cross build carries `default.gembox`, and with `mruby-bin-mrbc`
in it, `CrossBuild#bind_mrbc_host` generates an mrbc build of its own
where the config declares no host (:712).  Every other cross config in
build_config/ is written that way.

Delete the block, as 1520c97 did.  On master, run from a clean tree, the
config does not get past it: the host build defines `MRB_USE_READLINE`,
which mirb has not taken since 527018c, and aborts before the cross
target is reached:

    $ MRUBY_CONFIG=IntelEdison rake -j16
    ...
    CPP   mrbgems/mruby-bin-mirb/tools/mirb/mirb_completion.c -> build/host/mrbgems/mruby-bin-mirb/tools/mirb/mirb_completion.pi
    mrbgems/mruby-bin-mirb/tools/mirb/mirb_completion.c:45:10: error: #include expects "FILENAME" or <FILENAME>
       45 | #include MRB_READLINE_HEADER
    rake aborted!
    $ ls build/
    core2-32-poky-linux  host  repos

Without it, the same run reaches the cross target and stops where a
machine without the Edison SDK has to stop, on the cross compiler, and
build/host is not created:

    $ MRUBY_CONFIG=IntelEdison rake -j16
    GIT   https://github.com/pbosetti/mruby-hs-regexp.git -> build/repos/core2-32-poky-linux/mruby-hs-regexp
    CPP   mrbgems/mruby-bigint/core/bigint.c -> build/core2-32-poky-linux/mrbgems/mruby-bigint/core/bigint.pi
    sh: 1: /opt/poky-edison/1.7.2/sysroots/i386-pokysdk-darwin/usr/bin/i586-poky-linux/i586-poky-linux-gcc: not found
    rake aborted!
    $ ls build/
    core2-32-poky-linux  repos

The mrbc the cross target compiles its Ruby with is the generated one,
built with the host gcc into the directory that mechanism names:

    $ MRUBY_CONFIG=IntelEdison rake -j16 "$PWD/build/mrbc/default/bin/mrbc"
    ...
    LD    build/mrbc/default/bin/mrbc
    $ ls build/
    core2-32-poky-linux  mrbc  repos

The cross target itself still carries `MRB_USE_READLINE`, and will fail
on the same mirb line once the SDK is present; that is the target's own
define, out of the scope of removing a host build that duplicated it.