Symtab: Remove one copy of symbol size computation code

Summary:
The implementation in CalculateSymbolSizes has been made redundant in
D19004, as this patch added another copy of size computation code into
InitAddressIndexes (which is called by CalculateSymbolSizes).

Reviewers: clayborg, jasonmolenda, tberghammer

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D56132

llvm-svn: 350384