src/poky:webkitgtk: fix do_configure error on beaglebone-yocto

* According to latest comment [1] and the mentioned pull request
[2], build an ENABLE(WEBASSEMBLY) && !ENABLE(JIT) configuration is
supported, so original issue already fixed in current version, the
EXTRA_OECMAKE setting is not needed anymore.

* This EXTRA_OECMAKE setting causes following configure error on
beaglebone-yocto, remove the setting to let the configure process decide
the configuration:
CMake Error at Source/cmake/WebKitFeatures.cmake:312 (message):
  ENABLE_JIT conflicts with ENABLE_C_LOOP.  You must disable one or the
  other.

[YOCTO #15254]

[1] WebKit/WebKit#17447
[2] WebKit/WebKit#17688

(From OE-Core rev: 26a34fd71659e32e56ccc4f23e79aa62f4bc062d)

Signed-off-by: Changqing Li <changqing.liwindriver.com>
Signed-off-by: Richard Purdie <richard.purdielinuxfoundation.org>