xgettext: Optimize away a memory allocation.

xgettext: Optimize away a memory allocation.

* gettext-tools/src/x-c.c (phase5_get): Allocate the mixed_string_buffer on the
stack, not on the heap.
* gettext-tools/src/x-csharp.c (phase6_get): Likewise.
* gettext-tools/src/x-javascript.c (phase5_get): Likewise.
* gettext-tools/src/x-python.c (phase5_get): Likewise.
* gettext-tools/src/x-vala.c (phase3_get): Likewise.
* gettext-tools/src/x-rst.c (stringbuf, parse_string, extract_rsj): Allocate the
mixed_string_buffer statically, not on the heap.
* gettext-tools/src/xg-mixed-string.h (mixed_string_buffer_alloc,
mixed_string_buffer_done): Remove declarations.
* gettext-tools/src/xg-mixed-string.c (mixed_string_buffer_alloc,
mixed_string_buffer_done): Remove functions.