Revert "Add bfloat16 data type"

[luotao1]

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Reverts #25402

Liunx_nightbuild_python36_474.log

Maybe it is related with GCC version. The GCC version in log is 4.8.5

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[luotao1]

@wozna @lidanqing-intel Please fix the compile error in gcc4.8.5

[luotao1]

保存 “block.txt” [83/83])

2020-09-03 15:39:14 ****************

2020-09-03 15:39:14 0. The error message you wrote in PADDLE_ENFORCE{_**} or PADDLE_THROW does not meet our error message writing specification. Possible errors include 1. the error message is empty / 2. the error message is too short / 3. the error type is not specified. Please read the specification [ https://github.com/PaddlePaddle/Paddle/wiki/Paddle-Error-Message-Writing-Specification ], then refine the error message. If it is a mismatch, please request chenwhql (Recommend), luotao1 or lanxianghit review and approve.

2020-09-03 15:39:14 The PADDLE_ENFORCE{_**} or PADDLE_THROW entries that do not meet the specification are as follows:

2020-09-03 15:39:14 PADDLE_THROW("Unknown data type");

[chenwhql]

LGTM to PADDLE_ENFORCE

[luotao1]

@wozna @lidanqing-intel fixed PR in #26985 by @iclementine

[luotao1]

@wozna @lidanqing-intel fixed PR in #26985 by @iclementine