gdb/doc: Remove duplicate description of lookup_global_symbol

gdb/doc: Remove duplicate description of lookup_global_symbol

In this commit:

  commit 086baaf1346f07acfb6708e8c6cb79274241488b
  Date:   Tue Oct 15 16:18:26 2019 +0100

      gdb/python: Introduce gdb.lookup_static_symbols

A duplicate description of gdb.lookup_global_symbol was accidentally
added.  This commit corrects this mistake and removes the duplicate.

gdb/doc/ChangeLog:

	* python.texi (Symbols In Python): Remove duplicate description of
	gdb.lookup_global_symbol.

Change-Id: I4457b42cf05bde39e5c0ff39f168af919cad1255