Dropped generation of kiwix.pc

Below are the contents of libkiwix.pc and kiwix.pc files generated using
the new and old approaches, respectively, for native_dyn and
native_static configurations:

```
$ cat BUILD_native_dyn/INSTALL/lib/x86_64-linux-gnu/pkgconfig/libkiwix.pc
prefix=<REDACTED>
includedir=${prefix}/include
libdir=${prefix}/lib/x86_64-linux-gnu

Name: libkiwix
Description: A library that contains a lot of things used by other kiwix programs
Version: 14.0.0
Requires.private: icu-i18n, libzim >= 8.1.0, pugixml, libcurl, libmicrohttpd, zlib, xapian-core
Libs: -L${prefix}/lib/x86_64-linux-gnu -lkiwix
Libs.private: -pthread
Cflags: -I${includedir}

$ cat BUILD_native_dyn/INSTALL/lib/x86_64-linux-gnu/pkgconfig/kiwix.pc
prefix=<REDACTED>
libdir=${prefix}/lib64
includedir=${prefix}/include

Name: libkiwix
Description: A library that contains a lot of things used by used by other kiwix programs
Version: 14.0.0
Requires: libzim icu-i18n pugixml libcurl libmicrohttpd xapian-core
Libs: -L${libdir} -lkiwix
Cflags: -I${includedir}/

$ cat BUILD_native_static/INSTALL/lib/x86_64-linux-gnu/pkgconfig/libkiwix.pc
prefix=<REDACTED>
includedir=${prefix}/include
libdir=${prefix}/lib/x86_64-linux-gnu

Name: libkiwix
Description: A library that contains a lot of things used by other kiwix programs
Version: 14.0.0
Requires: icu-i18n, libzim >= 8.1.0, pugixml, libcurl, libmicrohttpd, zlib, xapian-core
Libs: -L${prefix}/lib/x86_64-linux-gnu -lkiwix -pthread
Cflags: -I${includedir} -pthread

$ cat BUILD_native_static/INSTALL/lib/x86_64-linux-gnu/pkgconfig/kiwix.pc
prefix=<REDACTED>
libdir=${prefix}/lib64
includedir=${prefix}/include

Name: libkiwix
Description: A library that contains a lot of things used by used by other kiwix programs
Version: 14.0.0
Requires: libzim icu-i18n pugixml libcurl libmicrohttpd xapian-core
Libs: -L${libdir} -lkiwix
Cflags: -I${includedir}/
```

The notable differences are:

- libdir changed from `${prefix}/lib64` to `${prefix}/lib/x86_64-linux-gnu`

- for native_dyn configuration Requires.private is used

- pthread has appeared in Libs/Libs.private and/or Cflags

- version information was added to the libzim requirement (however,
  for some reason it states >= 8.1.0, which is a deviation from the
  specified version restriction of ['>=9.0.0', '<10.0.0']; I was
  unable to find out where it comes from).

meson.build

@@ -74,20 +74,6 @@ if get_option('doc')
   subdir('docs')
 endif
 
-pkg_requires = ['libzim', 'icu-i18n', 'pugixml', 'libcurl', 'libmicrohttpd', 'xapian-core']
-
-pkg_conf = configuration_data()
-pkg_conf.set('prefix', get_option('prefix'))
-pkg_conf.set('requires', ' '.join(pkg_requires))
-pkg_conf.set('extra_libs', ' '.join(extra_libs))
-pkg_conf.set('extra_cflags', extra_cflags)
-pkg_conf.set('version', meson.project_version())
-configure_file(output : 'kiwix.pc',
-               configuration : pkg_conf,
-               input : 'kiwix.pc.in',
-               install_dir: get_option('libdir')+'/pkgconfig'
-              )
-
 pkg_mod = import('pkgconfig')
 pkg_mod.generate(libraries : [libkiwix] + extra_libs,
                  version : meson.project_version(),