Removed deprecated Doxygen option.

docs/Doxyfile.in

@@ -329,22 +329,6 @@ INLINE_SIMPLE_STRUCTS  = NO
 
 TYPEDEF_HIDES_STRUCT   = NO
 
-# The SYMBOL_CACHE_SIZE determines the size of the internal cache use to
-# determine which symbols to keep in memory and which to flush to disk.
-# When the cache is full, less often used symbols will be written to disk.
-# For small to medium size projects (<1000 input files) the default value is
-# probably good enough. For larger projects a too small cache size can cause
-# doxygen to be busy swapping symbols to and from disk most of the time
-# causing a significant performance penalty.
-# If the system has enough physical memory increasing the cache will improve the
-# performance by keeping more symbols in memory. Note that the value works on
-# a logarithmic scale so increasing the size by one will roughly double the
-# memory usage. The cache size is given by this formula:
-# 2^(16+SYMBOL_CACHE_SIZE). The valid range is 0..9, the default is 0,
-# corresponding to a cache size of 2^16 = 65536 symbols.
-
-SYMBOL_CACHE_SIZE      = 0
-
 # Similar to the SYMBOL_CACHE_SIZE the size of the symbol lookup cache can be
 # set using LOOKUP_CACHE_SIZE. This cache is used to resolve symbols given
 # their name and scope. Since this can be an expensive process and often the