glsl/mesa: remove unused namespace support from the symbol table

glsl/mesa: remove unused namespace support from the symbol table

Namespace support seems to have been unused for a very long time.

Previously the hash table entry was never removed and the symbol name
wasn't freed until the symbol table was destroyed.

In theory this could reduced the number of times we need to copy a string
as duplicate names are reused. However in practice there is likely only a
limited number of symbols that are the same and this is likely to cause
other less than optimal behaviour such as the hash_table continuously
growing.

Along with dropping namespace support this change removes entries from
the hash table as they become unused.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>