Drop stale gee-0.8 build dependency

No source, vapi, or generated vapi references Gee; the built
libinvercargill-json.so has never had a libgee NEEDED entry because
the linker drops the unused -lgee the meson dependency injected.
Invercargill's own consumer-facing invercargill-1.deps no longer
lists gee either, so nothing in the chain pulls it. Removed the
dependency('gee-0.8') line from src/lib/meson.build and the
pc:gee-0.8.pc entry from MANIFEST.usm build deps so the manifest
matches what the build actually requires.

Verified: clean meson build green with identical NEEDED set;
meson test 1/1 OK; usm manifest validate green; usm manifest test
green (both with LD_LIBRARY_PATH scrubbed per fleet practice).

MANIFEST.usm

@@ -37,7 +37,6 @@
       "bin:g-ir-compiler",
       "pc:glib-2.0.pc",
       "pc:gobject-2.0.pc",
-      "pc:gee-0.8.pc",
       "pc:json-glib-1.0.pc",
       "pc:invercargill-1.pc"
     ],

src/lib/meson.build

@@ -2,7 +2,6 @@
 dependencies = [
     dependency('glib-2.0'),
     dependency('gobject-2.0'),
-    dependency('gee-0.8'),
     dependency('json-glib-1.0'),
     dependency('invercargill-1')
 ]