samples: net: http_server: Disable mbedtls

Sanitycheck cannot link the result qemu binary because it is too
large. As a workaround, disable mbedtls to get the linking pass.

.../real-ld: zephyr.elf section `noinit' will not fit in region `RAM'
.../real-ld: region `RAM' overflowed by 192 bytes
collect2: error: ld returned 1 exit status

Change-Id: I543571505ab9412db7eac101848c6032d1550d79
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>