librem5_defconfig: remove nxp media drivers

2 jobs for imx7_updates_20210615_mr in 64 minutes and 32 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #359511
librem5
build:aarch64

00:19:45

passed #359512
librem5:arm64
build:deb

01:04:32