cleanup: delete unmaintained Dockerfile

This Dockerfile has been broken for quite a while now, at least since
Rust is required for building fish. No one seems to have complained
about it being broken, so there is no point in keeping it around. The
`docker` directory contains several Dockerfiles which could be used
instead.

https://github.com/fish-shell/fish-shell/pull/12408#discussion_r2770432433

Closes #12435

Dockerfile

@@ -1,18 +0,0 @@
-FROM centos:latest
-
-# Build dependency
-RUN yum update -y &&\
-  yum install -y epel-release &&\
-  yum install -y clang cmake3 gcc-c++ make &&\
-  yum clean all
-
-# Test dependency
-RUN yum install -y expect vim-common
-
-ADD . /src
-WORKDIR /src
-
-# Build fish
-RUN cmake3 . &&\
-  make &&\
-  make install