pgsql: Remove duplicate definition of the compressed varlena header

Remove duplicate definition of the compressed varlena header

toast_internals.h described the header of a compressed-in-line varlena
a second time, as toast_compress_header, with its own TOAST_COMPRESS_*
macros to read and write the tcinfo word. varatt.h already describes
the same bytes as varattrib_4b.va_compressed and provides equivalent
macros that act as setter and getter of this information, as of
VARDATA_COMPRESSED_GET_EXTSIZE() and GET_COMPRESS_METHOD().

Let's remove the duplicate definition, and switch the TOAST code to use
the macros from varatt.h. There should be no behavior change.

Note that toast_compression.h is removed from toast_internals.h, added
to the C files where its declarations are required.

Author: Nikhil Kumar Veldanda <veldanda(dot)nikhilkumar17(at)gmail(dot)com>
Author: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/CAFAfj_FeJQZrKyVn7ur0aJx8iz8SR14NheVJNyc8JNoqtMX+og@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e27f3b2cad7759097407ad01ccaaca506179e346

Modified Files
--------------
contrib/amcheck/verify_heapam.c | 1 +
src/backend/access/brin/brin_tuple.c | 1 +
src/backend/access/common/detoast.c | 7 ++++---
src/backend/access/common/toast_internals.c | 3 ++-
src/include/access/toast_internals.h | 29 -----------------------------
src/include/varatt.h | 15 +++++++++++++++
src/tools/pgindent/typedefs.list | 1 -
7 files changed, 23 insertions(+), 34 deletions(-)