doc: remove avutil.txt

No useful contents. Also hard to keep up-to-date.
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/avutil.txt

-AVUtil
-======
-libavutil is a small lightweight library of generally useful functions.
-It is not a library for code needed by both libavcodec and libavformat.
-
-
-Overview:
-=========
-adler32.c               adler32 checksum
-aes.c                   AES encryption and decryption
-fifo.c                  resizeable first in first out buffer
-intfloat_readwrite.c    portable reading and writing of floating point values
-log.c                   "printf" with context and level
-md5.c                   MD5 Message-Digest Algorithm
-rational.c              code to perform exact calculations with rational numbers
-tree.c                  generic AVL tree
-crc.c                   generic CRC checksumming code
-integer.c               128bit integer math
-lls.c
-mathematics.c           greatest common divisor, integer sqrt, integer log2, ...
-mem.c                   memory allocation routines with guaranteed alignment
-
-Headers:
-bswap.h                 big/little/native-endian conversion code
-x86_cpu.h               a few useful macros for unifying x86-64 and x86-32 code
-avutil.h
-common.h
-intreadwrite.h          reading and writing of unaligned big/little/native-endian integers
-
-
-Goals:
-======
-* Modular (few interdependencies and the possibility of disabling individual parts during ./configure)
-* Small (source and object)
-* Efficient (low CPU and memory usage)
-* Useful (avoid useless features almost no one needs)