system.h: remove all CURL_SIZEOF_* defines

[bagder]

... as they're not used externally and internally we check for the sizes
already in configure etc.

[jay]

My opinion is CURL_SIZEOF_ for curl types is appropriate. Is there some reason not to have them, and couldn't this be addressed by just removing CURL_SIZEOF_LONG?

[bagder]

I want to remove the SIZEOF defines from there because:

1. They're not necessary
2. They're not used by any public header/functionality
3. They make the system.h larger than it has to be, both in total and per system

[bagder]

Rebased and fixed merge conflicts

[coveralls]

Coverage increased (+0.03%) to 75.08% when pulling 6ccbdd1 on bagder/system-h-remove-sizes into 75536e5 on master.

[coveralls]

Coverage increased (+0.01%) to 75.062% when pulling 4731802 on bagder/system-h-remove-sizes into 75536e5 on master.