fcobjs: Remove duplicate function prototypes

These are already defined in fcobjsghash.h which is generated from
fcobjshash.gperf. This has been working fine because we were detecting
the len of FC_GPERF_SIZE_T, but it's incorrect in some cases, such as
on Windows, and then we get build errors like:

```
In file included from ../src/fcobjs.c:33:
src/fcobjshash.gperf:28:1: error: conflicting types for 'FcObjectTypeHash'
 "aspect",FC_ASPECT_OBJECT
 ^~~~~~~~~~~~~~~~
../src/fcobjs.c:28:1: note: previous declaration of 'FcObjectTypeHash' was here
 FcObjectTypeHash (register const char *str, register FC_GPERF_SIZE_T len);
 ^~~~~~~~~~~~~~~~
In file included from ../src/fcobjs.c:33:
src/fcobjshash.gperf:192:1: error: conflicting types for 'FcObjectTypeLookup'
../src/fcobjs.c:31:1: note: previous declaration of 'FcObjectTypeLookup' was here
 FcObjectTypeLookup (register const char *str, register FC_GPERF_SIZE_T len);
 ^~~~~~~~~~~~~~~~~~
```

We don't actually need to auto-detect this value in configure if we
let the header define it.
11 jobs for fix-msys-gperf-build-error in 5 minutes and 32 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #10378778
meson android arm64 fedora

00:01:31

passed #10378776
gst-macos-10.15
meson macos

00:03:12

passed #10378773
meson-mingw-w64-build

00:03:26

manual #10378777
docker windows 1809 allowed to fail manual
meson msys2
failed #10378771
meson-shared-build

00:05:02

passed #10378772
meson-static-build

00:05:12

passed #10378774
docker windows 1809
meson vs2017 amd64

00:01:31

passed #10378775
docker windows 1809
meson vs2017 x86

00:01:30

passed #10378770
mingw-build

00:03:55

passed #10378768
shared-build

00:05:32

passed #10378769
static-build

00:05:09

 
Name Stage Failure
failed
meson-shared-build Build
mktexfmt [INFO]: exiting with status 0
ninja: build stopped: subcommand failed.
Uploading artifacts for failed job
Uploading artifacts...
build-*/meson-logs/*txt: found 1 matching files and directories

WARNING: prefix-*: no matching files
Uploading artifacts as "archive" to coordinator... ok
id=10378771 responseStatus=201 Created token=ZYC2NWEd
Cleaning up file based variables
ERROR: Job failed: exit code 1