RFR: 8225779: Remove unused CollectedHeap::cell_header_size()

[Per Liden]

CollectedHeap::cell_header_size() is no longer used and can be removed. 
This patch is a hg backout of [1] where it was introduced, plus the two 
followup bug fixes [2] & [3].

This is a clean backout, with the exception of light editing needed in 
two places, because:

1) The function Universe::narrow_oop_base() has since been renamed to 
CompressedOops::base().
2) The include guard CPU_ARM_VM_MACROASSEMBLER_ARM_HPP has since been 
renamed to CPU_ARM_MACROASSEMBLER_ARM_HPP.

Testing: Tier1-3 on all Oracle platforms

Bug: https://bugs.openjdk.java.net/browse/JDK-8225779
Webrev: http://cr.openjdk.java.net/~pliden/8225779/webrev.0

/Per

[1] http://hg.openjdk.java.net/jdk/jdk/rev/4ad404da0088
[2] http://hg.openjdk.java.net/jdk/jdk/rev/f642ede2eb07
[3] http://hg.openjdk.java.net/jdk/jdk/rev/18bd95c0e463