main/linux-*: remove redundant override of HOSTCC

apkbuild-lint complains about the use of HOSTCC in APKBUILDs:

IC:[AL6]:main/linux-lts/APKBUILD:66:prefix custom variable with _: HOSTCC="${CC:-gcc}"
IC:[AL6]:main/linux-lts/APKBUILD:67:prefix custom variable with _: HOSTCC="${HOSTCC#${CROSS_COMPILE}}"
MC:[AL31]:main/linux-lts/APKBUILD:66:variables must not have capital letters
MC:[AL31]:main/linux-lts/APKBUILD:67:variables must not have capital letters

The first two were fixed in Leo/atools!24.
But apkbuild-lint still complains about the capitalized variable.
Adding an exception for this in atools seems quite complicated.

It turns out overriding HOSTCC in the linux-* APKBUILD is actually
no longer necessary. abuild automatically sets HOSTCC correctly now
when cross compiling, and the APKBUILD will just set the same value again.
(see alpine/abuild@9be173c6)

We can just remove HOSTCC from the APKBUILD.
linux-lts still builds fine when cross compiling for aarch64 using bootstrap.sh.
7 jobs for !4127 with remove-hostcc in 38 minutes and 3 seconds
latest detached
Status Job ID Name Coverage
  Lint
failed #51578
x86_64 docker-alpine allowed to fail
lint

00:00:33

 
  Build
passed #51583
aarch64 ci-build docker-alpine
build-aarch64

00:37:28

passed #51584
armv7 ci-build docker-alpine
build-armv7

00:26:43

passed #51582
ppc64le ci-build docker-alpine
build-ppc64le

00:09:06

passed #51581
s390x ci-build docker-alpine
build-s390x

00:09:58

passed #51580
x86 ci-build docker-alpine
build-x86

00:11:34

passed #51579
x86_64 ci-build docker-alpine
build-x86_64

00:14:51

 
Name Stage Failure
failed
lint Lint
No job log