[WPE][GTK] Remove pkg-config use from FindLibBacktrace.cmake

https://bugs.webkit.org/show_bug.cgi?id=265813

Reviewed by Adrian Perez de Castro.

libbacktrace upstream does not distribute a pkg-config file. Apparently
some downstreams are doing this, but I'm not sure who, because this
software is not packaged in major distros like Debian or Fedora. It's
not likely to be, because it doesn't have any releases. Let's not
attempt to use a downstream-only pkg-config file. If pkg-config support
can be upstream, that would be great.

* Source/cmake/FindLibBacktrace.cmake:

Canonical link: https://commits.webkit.org/272292@main

Source/cmake/FindLibBacktrace.cmake

@@ -28,21 +28,12 @@
 # OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 # ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-find_package(PkgConfig QUIET)
-pkg_check_modules(PC_LIBBACKTRACE QUIET backtrace)
-set(LIBBACKTRACE_COMPILE_OPTIONS ${PC_LIBBACKTRACE_CFLAGS_OTHER})
-set(LIBBACKTRACE_VERSION ${PC_LIBBACKTRACE_VERSION})
-
 find_path(LIBBACKTRACE_INCLUDE_DIR
     NAMES backtrace.h
-    HINTS ${PC_LIBBACKTRACE_INCLUDEDIR}
-          ${PC_LIBBACKTRACE_INCLUDE_DIRS}
 )
 
 find_library(LIBBACKTRACE_LIBRARY
     NAMES backtrace
-    HINTS ${PC_LIBBACKTRACE_LIBDIR}
-          ${PC_LIBBACKTRACE_LIBRARY_DIRS}
 )
 
 include(FindPackageHandleStandardArgs)