Cosmetic: temp/*: fix comments on top of APKBUILD

Remove Alpine's maintainers and contributors from the APKBUILDs in temp
where we still had them. In general, we don't keep them there, because
we don't want them to get e-mailed about questions from the postmarketOS
aports. It is obvious that they were copy pasted from Alpine, with minor
adjustments.

Remove "Remember to increase pkgrel after changing _pkgver!" from
temp/mesa, because we have a CI check for that now.

[ci:skip-build]: cosmetic changes only

temp/mesa/APKBUILD

@@ -1,7 +1,4 @@
 # Forked from Alpine, so we can upgrade the version and build etnaviv and tegra
-# ---
-# Remember to increase pkgrel after changing _pkgver!
-# ---
 pkgname=mesa
 pkgver=9999
 _pkgver=19.1.0-rc1

temp/qt5-qtbase/APKBUILD

@@ -1,5 +1,4 @@
 # Forked from Alpine to use OpenGL ES2
-# Maintainer: Natanael Copa <ncopa@alpinelinux.org>
 pkgname=qt5-qtbase
 _pkgname=${pkgname/qt5-//}-everywhere-src
 pkgver=9999

temp/u-boot/APKBUILD

@@ -1,7 +1,4 @@
-# Contributor: He Yangxuan <yangxuan8282@gmail.com>
-# Contributor:
-# Contributor: Timo Teras <timo.teras@iki.fi>
-# Maintainer: Leonardo Arena <rnalrd@alpinelinux.org>
+# Forked from Alpine to build for pine64
 pkgname=u-boot
 pkgver=2019.04
 pkgrel=0